when I was that speed with yau really all I did was do solves over and over.
but are you having a hard time with last 4 center building? because I'd reccomend the half centers method for them Jakob (hutnyk or or something is his last name... I forget) has a pretty good tutorial on youtube for it. It makes it so much faster and easier trust me!
Also, I recommend the 3-2-3 method for edge pairing, but you could get pretty good with 6-2 edge pairing also (I use 6-2 edge pairing ocxasionally).
the 3x3 step is also a pretty big part of 4x4 so if your not that fast at 3x3, I'd try to work on that as well.
Other than that, just turn decently fast while looking ahead (Mainly try to look ahead for cross edges while finishing up first two centers, and look ahead to 3x3 stage at the end of edge pairing!) and use inspection wisely.
By following just these tips, you can probably even make it sub-1 or sub-50 eventually.