First of all: When you get strong you'll get attacked by everybody and their uncle. It's hardcoded into the game. So expect an attack from every neighbor and you'll probably be right.

Second: NEVER send a crusade unless you're sure of it's success! You lose influence and risk civil war that way.

Third: Get VI and the patch - it's a completely different game (and far better).

Fourth: I think you're right that spain will attack Flanders - it's a popular target. So be prepared. Form a dedicated defensive army of elite troops (longbows not as useful as arbalests for an extended confrontation) and one stack should be capable of defeating four, especially since the ai seems to like building crap. Once the first stack routs you have an opportunity to inflict crippling casualties. Make sure your navies have numerical superiority in all sea-squares.

Fifth: Let us know how it went