I had the same problem and it was the control unit. You can send the control unit into Raymarine to have it checked but if its the control unit they don't make the 4000 anymore so you'd have to find an old one on EBay or some place. I had mine replaced with a 6002 as I recall the number.

The 4000 is only the control unit and the flux gate compass (the compass can fail and be your problem but that is pretty rare). ..... the 60002 has a separate "brain" that has to go below deck. My local guy had one in stock (and on warranty) so it cost $1,200 installed. If your going the total replacement route you might want to consider the below deck  auto helm model... don't know the model number but it works much better than the 4000 or the 6000 models.

I have the same problem. I found the following links and plan to dig into my wheel mount in January. Have to get beyond the holidays before getting back to some boat projects. Hope this helps.

http://pages.suddenlink.net/kmielen/C320Docs/ST4000fix.htm

Following link is the service manual for the ST4000. Relevant info regarding the wheel mount begins on page 12.

 http://www.ganssle.com/jack/st4000.pdf

The wheel autopilots are just too small for our boats.  Moving to a below deck evolution meant I had an autopilot that actually worked.  Straight as an arrow and it's handled some serious seas.  

The following seas I have used it in would have destroyed a wheel pilot in a few minutes.

[quote=richie30] My ST4000 quit this fall, and just grinds away and doesn't hold a course. This happened day 2 of a 30 day voyage on the Great Lakes. I hand steered for 1032 km's!
It might be the belt?? not sure, having trouble finding parts.
Any thought on a repair or a replacement ?
Thanks
Rich[/quote]

Sounds like it may indeed be the belt.  Ours stripped out while we were in the midst of our summer vacation cruise -- on an overnight sail, of course.  I discovered that apparently Raymarine no longer stocks parts for this autopilot.
However, I found a company that makes them to order (and overnighted it to me):
Polybelt Design, Inc., 26 W. First St,Clifton, NJ 07011 USA,973-340-1390,800-859-7659

(fax) 973-340-7444  info@polybelt.com (link sends e-mail)  
www.shop.polybelt.com/ (link is external)

Its their item number BAT5-750D169  ( D169 Belt for Autohelm Raymarine Autopilot ) $19.
Probably a very good idea to keep a spare one on board...