What 4800 champ should I buy?

Vladimir fulfills both requirements pretty well, 4800 ip, high damage, and gets very tanky even just building straight damage.

If you don't mind having a crappy laning phase and playing "farm the minions" for the first 15-20 minutes of the game you may like Nasus. He's tanky and when his Q stacks are high enough is a major damage dealer. He's 1350 though last I checked.

I think Galio is in the 4800 range. As is Jarvan IV, Lee Sin, Nautilus (I think), Shyvana, Rumble, Renekton, Skarner, Swain and Vladimir. My best bet is that you will end up liking Swain or Vladimir or Jarvan IV. Vladimir has an immense late game and Swain has a very strong early game where his late game isn't bad either. Both can be played either top or mid lane. Jarvan has strong dueling capabilities and is pretty good at initiating for his team too.

What role do you like playing?

Trundle any day. He's tanky as heck and loves to go toe to toe with other bruisers and tanks. He is one of the strongest dueling fighters in the game (top 3-5 at least), has very good DPS, scales harder than most champs while still being a major bully in non-ranged matchups, is one of the best solo tower killers in the game right after Nasus, and is a great disruptive frontliner and anti-tank for team fights.

He is nearly impossible to force out of lane because of his immense sustain and is probably one of the few champs that can reliably pull himself back into a losing game simply by ulting the enemy tank and using their own stats against them (basically, even if he is incredibly far behind, his health and resist steal guarantees at least a level playing field, if not a large advantage to you). He is also great at clearing the jungle, and although his ganks aren't amazing, they are still effective if you use your pillar well, and you can still become a major split push threat even if you don't have a huge early game presence (seriously, I was jungle Trundle one match, and within 2 minutes I took 3 top towers and the inhib while my team postponed enemy backs. We did the exact same thing a couple minutes later and I solo back doored the win within 7 minutes of taking the first top tower).

His only real problem is that he has pretty limited CC, can be kited by ranged/hyper mobile champs easily, and there is the slightest risk that you can screw over your team with a badly placed pillar.

While Trundle is hands down my favorite, other tanky damage threats that you may like include Vlad, Galio, Volibear (epic raid boss imo, fantastic champ), Wukong, Irelia, and Skarner, all of which are able to be extremely durable while having excellent (sometimes over the top) damage.

Those are all the damage-dealing, yet tankier, 4800 champions I can think of.

Renekton