Have you tested the PCC on a 4" radius? I only have experience with Bachmann's PCC but on mine (several years ago) it wouldn't turn a 4" radius. I'm not sure what the tightest radius it would run is as I was trying for 3 1/2" or tighter and do recall that I could not get even close to that. I seem to think it was around 7" but couldn't swear to that. Might be worth a test if not already done.