I think that could be highly dependent on implementation. If the Avengers just find Galactus and then stand idly by while he does all the heavy lifting, yeah, that wouldn't be very satisfying. But if, for example, they use Galactus as the distraction that gives them the opening they need to defeat the otherwise undefeatable, Thanos, I could see that working.
My thing with Galactus is I would like to see him as the huge threat they spend the next 10 films building toward. They could potentially do that with a brief appearance in A4, but my biggest fear is IF Galactus does show up, I don't want them doing anything that could jeopardize an otherwise potentially very satisfying slow, steady build toward a confrontation with Galactus in the future.