In online play the all-in after the flop is taken for a bluff a lot, so even though you had a strong hand a better move would be a nice bet of around pot size or twice the pot. I am kind of against the 3/4 pot size bet because that is more of probe/continuation bet size, a feeler bet to take the pot down with nothing but you have a hand and you want to get some value with it. You are not giving anyone pot odds to call, and most people take this sized bet to mean you have something, overbetting the pot like that looks like a weak move most of the time.
If you get called with that bet, normally you would be happy but with 2 spades out there you would have to bet again after the turn to make the other person have to really pay for that last card, however I would still not go all-in. Freerolls are funny in that the action is always pretty loose, not quite as bad as play money games but no one minds so much if they bust out since there is usually another freeroll starting up shortly after. So it may have been the case that no matter what you did in that hand, the person with A4s was going to stick around to the end.
