My buddy's HT4 wouldn't steer and it was due to the steering being out of adjustment. We adjusted one spot-on and the other requires us to remove the fuel tank so we can unseize the adjuster. Try that first (manuals are available in the library here) and see what you get. It's quite easy to overshoot the adjustment but if the levers themselves are relatively free (spray some lube), the you can feel when the actual clutches start disengaging to help you get in the ballpark. If you adjust (in both directions) and it doesn't seem to help, then your clutches might be stuck. If that's the case, then adjust them right up so that they will disengage the most, then drive around with your feet jammed hard on the brakes - make them smoke. The idea is to heat up the hubs to hopefully expand the components to make them "crack open". If that doesn't work, another tactic is to fill the steering clutch compartments with diesel up to just below the bearings and let them sit for days/weeks/months, rotating the clutches so that all the surfaces can sit in the fuel, and then try the braking approach again. If that doesn't work, you might be up for disassembling it. That is guaranteed to work but usually costs money as well as time : )

The main transmission clutch has a brake to stop the transmission input shaft turning which prevents grinding the gears. The brake is activated by pushing the transmission clutch lever as far forwards as possible. Maybe this will help?