Even if you never touched a 3x3 at all for the next year, you would probably improve at it to some extent if you practiced 4-7 a lot because, as you say, they use CFOP too. You can't really give a limit on your advancement on 4-7 based on your current 3x3 speed, as your speed on the 3x3 stage of reduction will naturally improve while practicing 4-7.
Anyway, I average 12-13 on 3x3 and can do 40 second averages on 4x4, and I still have a lot of room for improvement even outside of the 3x3 parts of the solve. So if you want to improve at bigcubes, practice bigcubes and don't worry about your 3x3 speed right now.