The TD9 just does more. It has more kits and more inputs/outputs, more recording capability (the TD4 just has a quick record and playback function). I have never hooked up either to a computer so I can't speak much to their capabilities there. I would look more at them on the website. I know the TD9 already has a USB port (you have to buy a midi to USB cord for the TD4).
I think the TD9 has something like 4 or 5 TIMES more instrument sounds than the TD4. I still think the TD4 has pretty much 95% of what I am looking for - I have never been upset because its missing an odd kit sound or two. If you are looking for really eclectic sounds, you might want to try the TD9. Otherwise the TD4 has everything I need - light, heavy, studio, arena, vintage, trashy, hip-hop, jazz, brushes, regaee, etc. There are 25 kits total, and 125 different instrument sounds on the TD4.
I am not sure what you mean by "better sound" as its all digital, so the sound isnt really better between the two. In fact, they share many of the same kit sounds.
I am not sure what you mean by adjusting the size and tuning of the triggers. The drum pads can physically be moved. They each have 6 screw "lugs" that can be tightened to affect rebound when you hit them. You cant really adjust the size of the pads themselves, short of switching them out with different pads, which should be possible with both kids.
The sounds themselves can be tuned, muffled, given reverb, and adjusted for volume and sensitivity. Within each kit you can swap out the instrument that each pad triggers. For example. you could adjust the floor tom pad to be a 14" maple, a 16" maple, an 18" vistalite, or even a cow bell or ride cymbal. I have one kit saved in which the pads triggers a cow bell, shaker, tambourine, and snare.
$1200 is a great deal for a TD9 if it is in good condition, especially with the pedal (you still need a great pedal for e-drums) I imagine its the new model right? The new models included a lot of upgrades. E-drums are amazing since they are pieces of sensitive electrical equipment designed to be banged on heavily by drum sticks. However, sometimes they break and wear out, so make sure the check the kit thoroughly first. Probably the most common issue with recent Roland kits is clogged cymbal triggers. As the cymbals are banged, a bit of the rubber wears away inside the trigger and clogs up the trigger. The issue can be fixed by opening up the cymbal and cleaning the trigger, but you have to be careful doing it. I have only had it happen to my hi-hat (it triggered inconsistently when playing fast patterns), but it was easy to fix when I pulled it open. Roland would not honor my warranty because i bought it used. Just something to be aware of.