guess what, I've used -4 and -3 without problems. I prefer -3 because its smaller, easier to work with, looks better and the ID of the -3 is about the same size as most factory turbo charged cars oil lines.

 

Aside from physically measuring the oil feed pressure on an OEM, matching ID with an OEM would get you close enough. I personally set mine with a T-valve to not exceed 40psi at high revs, but still register at idle.

 

My kits come with a -4. I doubt you would see any different pressures from either of the two. Flow, maybe, but not pressure.