I just got a set of -4/-3 lines and wow, they have a lot bigger ID than the straight -3 lines do! Surprised to see that as the only difference you can see on the outside is that the fitting on one end is the bigger -4 size.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax