So I'm trying to figure out which champion I should buy next to best benefit my team and I'm stuck between one of the following:

Leona
Thresh
Maokai
Rumble

I picked these four because Leona and Thresh are two of the most popular support champions right now, Maokai because I come across many who enjoy jungling with him (and occasionally supporting), and Rumble because I lack an AP top and discovered this problem during a game sadly.

Out of these four, I'm leaning more towards Leona and Thresh. Anyone mind helping me decide?

I am sorry but if you don't know for yourself what style you want, then that means you have not played the game enough and should play more as it would help you decide. Why be so hasty to buy something when you clearly don't even know what it is yourself you want.

Use your own problem solving skills compare them on wiki. For the most part play the damn game.

Darn, out of those four I've only played Maokai. Sadly, I did not care for the Twisted Treant. I'm sure he's able to be played well, but I haven't seen it yet.

Though I haven't played the others I think that Thresh is the strongest of the four, I have been supported frequently by Thresh players and each of his abilities has the potential to create devestating combinations with any champion in the game!

LeBlanc!

i would try to focus on one role... leona is not that popular right now, and thresh outclasses her in lane. rumble is a solid top, even though he is nerfed, he is still a really strong champion. maokai i dont think is as strong as other junglers. so basically thresh or rumble, both are fun to play.