to me given the info I have at hand it sounds like you changed from a .48 to a .63 housing and then the problem appeared, the problem as you describe it sounds like your getting surge, I would go with for some reason that is the case but I dont know why it would surge at such low boost, I may be talking bollocks but is there any restriction in your inlet system? are you getting full throttle etc, it seems that something must be stopping your engine consuming all the air that the turbo is trying to give it at 4000rpm, most odd. Obviously try the fuel filter etc just incase, also have you tried asking someone like Stu who supplied your chip if he has any ideas why it might be doing this? he obviously has a wealth of knowlage on the subject of tuning cossies and may have come across this one before.