Looking at a Trackmaster 4t4, have not seen a 601, but from the outside they look similar. How is the 4t4 driven? it does not have the familiar OC differential in the back. There are two drive shafts going directly to each sprocket, but what drives the shafts is hidden from view. Feedback would be great! Reliability? Ground Pressure? parts avail? price range? DO they manuever better / worse than a spryte? Anyone own one?
The 4T4 would be a great purchase. Basically, the 601 and the Trackmasters are the same with one big exception... the differential vs. a power distributor. People have done a few different things as SnOps said... drop a diff in of various makers (OC's or Bombi's or other...) and removed the power distributor completely. The power distributor is comprised of a 24-clutch-disk system which isn't that tricky to rebuild... plenty of folks have done it and rebuilding isn't that nasty. A pack of 24 clutuch disks will run you $350. Don't buy them off eBay as they're marked up quite a bit by folks selling there. On the other side of the coin the drop-boxes in the back for the two driveshafts are easier to replace than any of the differential gear. The diff for a 601 is pricey and they are hard as hell to find. To replace some of the control mechanisms that drive the 601 are pricier than the 4T4... those items specific to the differential. Similarly, to replace or do any diff work on any diff-machine is very pricey. One of the reasons for going to the OC15 diff on the 601 was special care and feeding of the dual-shaft design. It has it's fragilities and needs a bit of fine-tuning with the dual-shafts but still is plenty robust unless you are pulling a drag or doing timber work with it...which some were used for which goes to show you that it is beefy. They were originally ATV's more than snowcats so none of them float like a proper cat. The Sprytes do much better in powder. The Trackmaster nor the 601 do well in powder because they're too heavy with too little track. So, if you're going for powder then that shoudl be your biggest 'no-way' sign ahead.
The track for your 4T4 are different than the 601 but nothing too different...slightly more maintenance on the 4T4 than the 601 due to complexity of the grouser design (if they get bent, or need replacement, etc.). Boggies are different between the two but easier to refresh on the 4T4 than the 601.
Other than that they're the same body, frame, suspension, etc....
If you don't want it, let me know. It's probably too far away for me but otherwise I'd be interested in picking it up as I have a 4T2, a 4T10, and a 601 but I've been waiting to find a 4T4. PM me if you decide not to pick it up. I have a friend out West that's kind enough to let me store cats until I fill up a semi to haul East which is far cheaper than a one-off.
Whatever you do, don't let SnOps talk you out of it... he's buying every piece of Thiokol equipment on the planet.

I don't know what the hell he's doing with it all... probably goes back to that snow-pussy photo.

He has the ability to sniff-out deals underneath rocks and hard-to-find places.
The price is really heavily dependent on condition (go figure), although we just saw that timber-tracked vehicle for sale up in AK for $20,000 and it seems it's warranted based on another sale. Whatever... I think it's about interest. Hardly anybody wants the old Thiokols and SnOps is right... based on parts availability... but alot can be swapped/replaced or is just Ford-stock. All until you get to either the diff or the power distribution unit.
Hope this helps.