4mm vs 5mm doesnt mean shit really. rider ability, tires, gearing, swingarm..just general setup will play a huge part in how a bike runs and gets out of the hole.. ive seen 350 motors flat out spank 4mm motors, so dont rely on what you saw too much.
most people are going to tell you that the 4mm is the common way to go. its reliable, easy to get parts for and its very cost effective.
i think you need to talk to some other builders. i dont know why any builder would be recommending a 5mm motor over a 4mm motor..