I played IV on PC and really enjoyed it (though didnt play it much online) and especially liked the modding options.

If you want to play V when it comes out you'll need either a PS4 or PC anyway (im not even sure about the PC?) which might be worth considering

Sticks are usually platform specific and not game specific, so if you get a PC stick, it should work with SFV (PC Version ofc)

SF5 is only on PS4 and PC. If you want to stay current on the community, 360 needs to go. As for ps4, it's getting but doesn't currently have sf4. You'll have to use PS Now atm is you migrated.

I'd wait to see if the PC version of SF5 is any good since you wouldn't have to buy anything to do it that way.

On the PS4, there is a lot going on where they are perhaps opening the door to PS3 fightsticks, which means that you could potentially pick up an older model but still awesome fight stick for your PS4 and SF5 down the line for cheap. There are also ways of modding the sticks to change compatibility from one platform to the other, but I've never done that myself.

Again, no confirmation AFAIK about PS3 fightsticks being possible with SFV on PS4 just yet, but there is at least one game, and a petition is there for MKX as well to get PS3 stick support. The driver that makes it work is royalty-free, so there's not much stopping Capcom from getting onboard, unless they really want to help sell PS4-only sticks by deliberately locking out PS3 sticks lol.

Are fight sticks better than those official Genesis like SF controllers? Personally, there's no way I could be as good with a stick as I could with one of those controllers. Those d-pads are amazing.

The Community is not nearly as good/busy on the PC.

Street fighter V will be here in 2016 at the absolute soonest. I can't really say that Ultra Street Fighter IV would even be much help, in terms of prepping you for SFV.

SFII & SFIII were vastly different from each other, adding dashes, EX attacks, universal overheads, and parries. As well as SFIII & SFIV. So, really, it doesn't matter which Street Fighter game you play. SFV will be different from all of them, while keeping only basic fundamentals.

Any stick you bought to PC will works on any fighting game... so you will be covered on PC with SFIV and SFV after.

Now for console the only option is PS4 for SFIV and SFV and any stick supported for SFIV will be supported to SFV.

The real issue is if old sticks for PS3/360 will works on PS4... but I'm sure any stick supported on PS4 will works on PC too if you want a stick to works in both then it is better to see the PS4 compatibility first.