Is Calibans's behavior in the presence of Trinculo and Stephano in Act 4 different than in earlier acts?
Yes, though to be honest, I'm not sure it is due to the presence of Trinculo and Stephano as much as the change comes from other sources. By that I mean, Caliban acts very different here. He was surly and threatening earlier, angry at Prospero for the old injury of stealing the island from him. Here he is much nicer, essentially playing the fool, but is it because of Trinculo and Stephano? I say no. He is clumsy, funny, and foolish, but that's because the sailors get him drunk. He's shown to a great innocent here. His drunkenness shows that he really doesn't deserve to rule the island.