I used a TSC stores PHD on my 4100 last year to drill about 30 holes to 40" in clay. It worked fine. In fact, it was an improvement over a larger tractor I used the year before, for reasons of manuverability.
Note, if you get the auger stuck in clay, shut off tractor, unbolt auger, unscrew with pipe wrench. You will not have enough 3pt hitch lift to pull it out. Withdraw from hole often during the drilling, as soon as flighting appears full, to reduce chances of getting stuck.