4800 IP Champion purchase decision
I need to know which champion to purchase whether it be wukong or riven. Please tell me from personal experience and if you think there is a stronger champion for 4800 IP then please tell me below.
I need to know which champion to purchase whether it be wukong or riven. Please tell me from personal experience and if you think there is a stronger champion for 4800 IP then please tell me below.

Rivens too hit or miss. You might not like her. Get wukong. Or save up for a 6300 ip champ :^)

they are apples and oranges. Riven has a lower skill ceiling, you could try her. But Wukongs fairly cool with his clones and stuff
Go the monkey, great champ snowballs hard and can carry games. Also you can face stomp a riven once you get good at him. I'm gold 5 and the monkey is still one of my favourites and carried my but out of bronze. All hail them jukes. 
Get Riven between those two. She's overall better if fed. I will say however Wukong has more variety, he can be played jungle as well as top.
Riven is almost strictly top although she can jungle, just not as good as Wukong can.
So for variety get Wukong, for a better player experience, and overall better performance, get Riven(again if she's fed, she becomes pretty much almost the most Op champion in the entire game, otherwise she sucks, but be prepared to learn your top lane opponents because she can struggle against quite a few).
Update: Bought Riven. I prefer energy over mana use any day and she seems really fun to play as a top laner.
Pick Riven if you want to be hated by everyone.
Pick Wukong if you want a champion that is OP enough to get you to diamond but no one seems to realize he's OP.
Renekton is 4800 isn't he? Sounds like you play top lane, he's solid