Stuttering could sound like it is going into rich&retard mode, this can usually be if you are running too much boost and it'll hold or hesitate for a second before picking back up..
- are you running stock boost..
- what other mods have you got..
- which engine exactly is it?
have you had a chance to plug in a consult cable and check that all your sensors are doing ok + maybe raise the idle RPM to see if it makes a difference, don't believe this is permanent - however it would give you an idea..
also try using calumsult which lets you record the data, go for a spin than check back through the ignition timing so on and so forth, it might help isolate the issue at 4,000 rpm..