Well, you can use yoshinator's "half-center" method to improve the centers.How to actually get Sub 1:40 on 4x4?
I am awful at 4x4. I use yau, and I get 2 minutes average on my Aosu. I average 22 seconds on 3x3 so I'm really surprised. My centers are extremely slow. How can I get faster at 4x4 in general?
If you are not already, learn 3-2-3 or 6-2 edge pairing.
And everyone says this, but lookahead is important. I'm being a hypocrite here, cus my lookahead is really terrible, but still.