The arena is plenty fine for IV and could serve you well for a while in III. I'd avoid investing too much more into it now, and if you do, resist the urge to throw up a load of cheap bleacher seats. Spend some time thinking about where you plan to get to in the game, how you want to get there, and what your arena should look like when you're there - and then prioritize the lower tier seats vs. bleachers if you want both. But make sure not to build seats you can't sell either.
