You should promote. The promotion bonus, increased TV money and attendance boost will generate enough money to improve your arena and roster to stay in the league. You've got a whole season to prepare for a possible relegation series, which should be enough if you're an active manager (which I think you are seeing you post in the forums).
Worst case scenario; you use your money to improve your arena and roster, end up relegating but having a high draft pick because of a bad season, which could end up with a nice trainee. When you're back in D5 you're surely stronger than the opposition because of your improved arena and roster.