is it worth using 4 lv 4 key spheres to get ultima?

how powerful is it compared to firaga / thundaga etc...?

I suppose that depends on your magic. If your magic is high enough - why not?
I think I used it... But Lulu hadn't got her ultima weapon, so it wasn't worth getting it.

By the way it also depends on how many lv 4 key spheres you got. In the end I was lacking those because I didn't find more. ^^"

You only need 3 level 4 key spheres to get Ultima, don't you?

You get level 4 key spheres by bribing Chimeras.
It depeneds wether you're just trying to beat the final boss, or if you're trying to complete the entire game.

By the time you have the stuff to get Ultima, you probably will have plenty of stuff more powerful than Ultima deeming it worthless (like Wakka's pure physical attack), but I guess you can go for it if you're trying to get everyone to learn everything, which is what I usually do.

The only time Ultima really helped me out was in beating Neslug. When he retreats into his shell and becomes impervious to physical attacks, I just had Yuna and Lulu doublecast Ultima. Other than that, I don't really remember ever needing it too badly.

Make sure you get ultima at very high magic because if you dont you
will end up using lots of mp just hitting 1000 damage

I personally found that ultima was nice...especially when you can copycat someone double-casting it for only 1 mp. Though, that said, right now all of my characters are doing 99,999 with their regular attacks. So, it just depends on where you are in the game.