what is the fastest way to improve reduction time on 4x4? Particularly edges. Do you think that using the metronome the way you use it for 3x3 f2l would work for edge pairing?
Using a metronome would probably help.
I'm assuming you use 2-pair.
When I did 2-pair I saw the pairs as 2 different pairs. The one that you paired up with just a Dw/Dw'/Rw/-Rw' was the base pair. The pair you made when you finished it was the complementary pair.
I looked for the base pair while doing the previous pairs, then while I was putting the base pairs in place I looked for the complementary pairs, etc.
Oh, & also, practice. I did an AO12 every day for about 2 weeks & went from 2:00 to 1:20.