I have a 4000 that I use as my main machine. Other than my serger, it's the newest machine I own. I love it. Other than doing a search online for repair issues, I'd say to really clean out the bobbin race/feeddog area. Stuff get's gunked up back in there, threads wrapped around. Take off the needle, the foot, the cover plates, take out the bobbin, the bobbin case... take out everything you can (do NOT loose the screws!)
Then get in there with a flashlight and really really look. raise/lower the dogs etc, turn the wheel to rotate hook... and find the pesky lint, dirt, threads that might be there. I've seen impacted built up gunk in 'new looking' machines before.
If that's not it.... maybe someone else has a better idea. It might could a broken pin on the bobbin case, a broken spring or whatever is connecting the feed dog mechanics to the rest of the moving world.
If you're afraid of taking it apart... do 2 things first. 1. is UNPLUG IT before you do anything. and 2. Take pictures of the machine and it's surfaces/parts before you dismantle it. Then you'll have a visual record of what goes where. I also use small sandwhich sized ziplock baggies to put parts from each area into (into sep baggies and marked with sharpie markers as to where they came from.) That's because it always takes me a week or two to finish my projects... lol.
How about other suggestions? Anyone?