I've done small 50 lb pigs and lambs on a spit several times, maybe 5 times I guess. This was a couple decades ago. It was a home made deal. I used a lawn mower tractor with a couple gear reduction transmissions and also pulleys to get it to a slow speed. Never did bigger than 50 lbs. You need a nice strong spit and you need to be sure there is nothing that wants to flop around including the torso itself which gets loose when it gets hot. You want to be sure it's secure on the spit, like rock solid secure because, again, it will loosen up when it gets hot. When the pig flops around, it's very hard on the motor. And finally, you want the entire thing to be BALANCED. That's the most important thing. If you can spin the spit with your fingers before connecting the motor, that would be good. So you'll need counter-weights. The 4 rpm speed on that Titan surprises me - I have a HD motor from Onegrill which is 27 watts and it only spins at 2 RPM which provides much more torque, and yet they only rate it at 85 lbs. Mine has a built in fan to keep it cool. I paid $160 for it, which was a sale price, and that was just for the motor alone.
I've bought several items from Titan in the past and I have no complaints. In fact, I think I've gotten some crazy good deals from them.
But I've never used my Onegrill motor to spin a pig. I now cook pigs with a La Caja China box which does a MUCH better job cooking a pig, and much easier. The whole pig is perfect and moist from end to end and you get perfect cracklin' too. It's just not as much fun to look at because you can't see it while it cooks! Good luck.