Sub 40 on 4×4 is pretty fast in my opinion. It cannot be achieved easily. With that being said, it is possible with enough practice and time. If you average around 10 seconds on 3×3, I’d say your 3×3 stage is going to be good enough to be sub 40. Since the yau method is the most popular and I too use it, I will give you tips and advice based on it.
There are 6 steps in the yau method.
- First 2 centres
- First 3 cross edges
- Last 4 centres
- Last cross edge
- Last 8 edges
- 3×3 stage
1. First 2 centers
- For 3×3, you plan out the entire cross and sometimes the first pair as well during inspection. In the same manner, try to plan as many of the first 2 centres as you can during the inspection time. Planning one whole centre and half of the second centre is good enough. At first, you can use as much time as you need to plan a centre and a half. Eventually, work on reducing that time.
- You should turn fast during this step. Aim for solving it in under four seconds.
- Watching a lot of examples solves from fast cubers and implementing their techniques into your solves can be beneficial.
- For most solves, you should be solving the first centre at the bottom or on the left/right side. You should almost never solve the first centre on the top.
- Your solutions must include finger-trick-friendly moves. Assuming you average around 10 seconds, I presume that you can make a good judgement on whether a solution is finger-trick-friendly or not. R, L, U, F, Rw, Lw, Uw and Fw moves are usually the best choices. You can occasionally use B or Bw moves. D and Dw moves can be difficult to execute, so it is better not to use them often.
2. First 3 cross edges
- During this step, you can turn slower because it can be hard to find cross edges from all over the cube. Aim for solving it in around 6 seconds.
- Each edge pair can be solved in 6 moves or less. Be efficient and find the best solutions possible.
- If you are right-handed, solve the cross edges towards the left side. This will allow you to solve the next step using right-handed moves.
- Do not solve the edges in the wrong order first and correct them later. Solve them in the right order from the beginning itself.
- As soon as you spot the two pieces of an edge pair, you should be able to solve it without thinking a lot. It’s just like how you solve f2l pairs on a 3×3. You can practice this by trying to solve it with your eyes closed. This is not something that you will be able to do immediately.
3. Last 4 centres
- During this step, you can turn moderately fast. It is easier to spot the pieces, because each piece has only one colour, unlike edges which have two.
- Make sure that the cross-edge slot is empty on the upper layer. Fix your left hand to the cross-layer, and don't move it. Use the other three layers to solve the centres.
- For this step, it is crucial to be very familiar with the colour scheme. You shouldn't have to think a lot about it. This can be achieved eventually just by doing more and more solves. You shouldn’t solve the centres in a specific order every time. Be flexible with your solving order.
- For most solves, do not solve the centres which are opposite to each other first. Doing this can slow down your next 2 centres.
- For the last 2 centres, try to come up with better solutions. By being efficient at this step, you can save 1-2 seconds depending on your current level. Try to figure out the best solutions by doing slow solves.
- Try to preserve 1×2 bars, but only when it is worth it. If it takes more than 2 moves to preserve a 1×2 bar, it’s better not to preserve it because it only takes 3 moves to make a 1×2 bar most of the time.
- Once you spot all the pieces of a centre, you should be able to solve it without looking at it. This will help you in looking ahead to other centrepieces. This doesn’t mean that you should spot all the centrepieces first and then start solving. While solving, if you solved a 1×2 bar and then saw the other two centrepieces, then you should be able to solve it without looking. That is when you should look ahead to other pieces.
- There is a method called half centres. In this method, three half-centres are solved by making 1×2 bars. The half-centres are attached to the cross side. And then, the rest of the centres are solved using only R and U moves. This method is worth learning.
4. Last cross edge
- You can turn fast during this step. This step shouldn’t take more than 3 seconds. Try not to waste a lot of moves. If you looked ahead, then you should know where the pieces of the edge pair are.
- You should be able to solve this without thinking a lot. After spotting the pieces, try to solve it without looking (while practicing).
Watch Yau Method Video - Explained by Aryan Chhabra
5. Last 8 edges
- 3-2-3 edge pairing is the norm for solving the last 8 edges on 4×4 these days. If you still use free slice, then stop using it and switch to 3-2-3 edge pairing. This is where we save a lot of moves and this is what makes the yau method faster.
a. First three edges
- While solving the last cross edge, you should be looking for the first edge pair that you want to solve.
- If there are solved edges in the middle layer before slicing, then don’t move them to the top layer to preserve them. Instead, slice and move on assuming you have paired one edge.
- If you find solved edges after slicing, there is no need to move them to the upper layer to preserve them. Just ignore them and move on. They will be preserved automatically.
- Get comfortable with inserting edge pairs into the back, left and right. Don’t always insert edges into one side.
- You will come across some bad cases for which you may not know the best solutions. Learn better solutions for such cases. You can find good solutions here.
b. Last few edges
- You can turn fast during this step as you progress because looking ahead is very easy.
- Learning the last 2 centres' algorithms will be very helpful. Cubeskills has good algorithms for this step.
6. 3x3 stage
- You should start looking for the first f2l pair while finishing the previous step. Turning can be difficult during this step because it does not feel like a normal 3×3. Doing timed solves for the 3×3 stage will be helpful.
a. OLL parity
- It is important to know how the OLL parity algorithm affects the pieces. By knowing that, you can force easier OLL cases after executing the parity algorithm and sometimes predict the OLL.
- If you have 3 edges oriented, then you can put the unoriented edge in the front and do the algorithm. This will give you an all edges oriented case, which is usually better. If you get one edge-oriented case, then you can put the oriented edge anywhere else other than in the front to avoid dot OLL cases.
- You can go even further by learning setup moves to skip OLL if you get a parity. However, this does not work for one edge-oriented case, and some of the 3 edges oriented cases. You can learn it from Jperm.net. (assuming you use Rw U2 x Rw U2 Rw U2 Rw' U2 Lw U2 Rw' U2 Rw U2 Rw' U2 Rw' for OLL parity)
For example, in the image below, you can skip OLL by doing L R' (parity) L' R.
b. PLL parity
- You can be smart during PLL parity and force an easier PLL case if you get a parity. The PLL parity algorithm (2R2 U2 2R2 Uw2 2R2 Uw2) swaps the front and back edges of the last layer. By using this knowledge, you can try to preserve as many bars as possible. For example, in the image below, if you execute the parity algorithm facing the green or blue side, you will get an F perm. But if you do it facing the orange side, you will get a much better T perm. There are more such cases which you can figure out on your own. Just try to preserve as many bars as possible.
- Some cases are not so obvious, for which you need to use setup moves or do the parity after performing a certain PLL algorithm. Jperm.net is a good source for all such cases.
- Watch a lot of examples solves from fast cubers and try to implement their techniques into your solves. This tip counts for every step.
- Transition times are very important. They should almost be equivalent to zero seconds most of the time. Many cubers often tend to pause between the first 2 centres and cross edges. Sometimes, when you finish edge pairing, it can be hard to suddenly switch to outer layer moves only for f2l. You need to practice that as well. You can try to predict whether there will be OLL parity or not while solving your last f2l pair.
- Drill your parity algorithms. It can be frustrating when you mess up because of the poor execution of parity algorithms.
- Alternate between efficiency training and turning training. Turning for 4×4 is quite different from 3×3, so good turning on 3×3 may not reflect into your 4×4 turning. You should learn to control the layers properly.
- Practicing big cubes like 5×5 and beyond can improve your look ahead.
- Having good hardware will be helpful.