Like already noted, there's no change to arena seat pricing in D4. But also, there's no maintenance cost for your arena. So in many respects, it's better to start early with the expansion, so that you can immediately start reaping the benefits.
Also, there's the promotion bonus to attendance, which lasts ~1/2 or slightly more of the season. Now, if you keep winning, it's not a big deal, because you'll keep filling your arena. But it's possible that you'll have consecutive home games during the first intraconference portion of the schedule, and then one less during the second part, at the end of the season. In that case, you'll certainly want to have already expanded your arena. Honestly, if you have 600K already right now, I'd put 300K of that into the arena, with something like 750 bleachers and 200 lower tier. That'll leave you money to pick up some veterans at decent prices after the playoff deadline comes. Next season when you're in D4, you can see how your team is doing, and either pick up some more players if you're stuggling, or plow the rest into the arena.
One suggestion is to check the 'attendance helper' under the 'team tools' after you log into the USA database (bb-usa.host22.com). Entering the number of seats you have (or will build) and switching to D4, you can get a decent idea of the relationship between seat prices and attendance. I'd suggest going a bit over the recommended ideal price, because that doesn't adequately account for the promotion bonus. That will help you reap more revenue from your arena.