I wasn't going to mention anything because your problem seems to be enough different than our old problem that it wouldn't be helpful for me to mention it. Anyway, our problem was similar to yours in that our 4120 was blowing heavy WHITE smoke (not blue), but right after startup...not after running for awhile. The mechanic said the John Deere engineers told him they were aware of this problem and they recommended replacing the coolant temp sensor. Sure enough, after he replaced that the problem was solved. What happened with ours was this. We bought it last winter. Ran fine all winter. Once it warmed up in late spring, many times after startup, the tractor would start running rough (and much quieter) and blow white smoke...power was drastically reduced. It you rev the engine it would blow heavy smoke. Most of the time letting it idle for 5 minutes or so would clear the trouble. Once it was warmed up, the problem was gone. I also found that if I noticed the muted engine noise (the classic diesel pinging sound was very muted) and white smoke, I'd shut it off immediately...then restart. After restarting the trouble was gone. Now that it's fixed, I have the normal 1 or 2 seconds of rough idle, a puff of black smoke and engine rev on startup...all normal and great.
I hope they aren't serious about these engines having problems like this on a regular basis. Ours only has 110 hours. I did notice the same engine on JD's web site is rated at a higher horsepower than what we have in the 4x20 series tractors. They must really be 'detuning' them quite a bit. Wouldn't it be nice to unleash the real power of these beasts? /forums/images/graemlins/smile.gif
Hope they get your tractor fixed soon.
EDIT: I just noticed on the very first page of your posts that you said there WAS white smoke. You really should have them try the coolant temp sensor. It worked for me. As crazy as it may sound...ask them to give it a shot. My dealer is Bowen Farm Supply in Gardnersville, KY if your dealer wants to call them and ask the service guys about my issue and the resolution. Mechanic is Nick. This is a rural dealer with lots of larger farm equipment and lots of experience. I've been very happy with their service dept.