Looking for a bit of advice.
I have an STI4, which is 2000 miles after a rebuild by a very reputable engine builder.
The car is running very well, but tonight I noticed something odd.
When I accelerate hard in 4th gear (only 4th mind) with about 6000rpm on the clock I start to get a small amount of grey smoke coming out of the exhaust.
Now the car was trundling around being run in for a few weeks, and I've only just started to use the upper rev band in the last 500 miles. I only get the problem in 4th, and haven't tried to replicate the problem, in case it's serious. Anyway since it doesn't do it in 3rd or 5th any ideas?
I suppose it could just be muck in the exhaust that needs a good clean out and maybe 4th gear resonation shifts it better than 3rd or 5th.
Any ideas?

BTW: MAF Sensor has been recently replaced and tested and there was no det detected on the dyno run when it was last setup.

Any advice appreciated.