If you have the opportunity to promote, it is almost always best to take it. D IV isn't that much harder than D V, and if you manage your team well, you can certainly manage to avoid relegation. If you're going to promote though, start building now. The promotion bonus is significant so build as much as you can as soon as you can, and jack up your prices a fair amount to ensure maximum income. Worry about players at around the start of inter-conference play.