Two things:
1) USA D4 typically isn't as much of an increase in difficulty as you might expect. At least, it's not so hard to avoid relegation. Winning a league can be tough, but sticking in the league isn't too bad. And the money is much better in D4. So, go for promotion. You'll still be able to train youngsters and build your arena if you do.
2) if you for sure don't want to promote, it's at least worth it to try and get to the finals. Even if you lose, you'll get a nice boost in season ticket holders, which will improve your gate receipts and merchandise money.