Why is hamlet able to act in scene 4 but not in scene 3? what does this tell readers and viewers about hamlet’s character?

In act III, scene iii, Claudius is kneeling in prayer when Hamlet finds him. He doesn't kill him, even though he has the perfect opportunity, because "Now might I do it pat, now he is praying;/And now I'll do't./And so he goes to heaven;/And so am I revenged. That would be scann'd:/A villain kills my father; and for that,/I, his sole son, do this same villain send/To heaven." 

In the next scene, Hamlet mistakes Polonius hiding behind the arras for Claudius. Unlike, scene iii, he's not in prayer, so there is no similar worry about whether he'll go to heaven. 

Throughout the play, Hamlet seems to have this inner conflict over whether revenge is the 'right' thing to do. And what comes after death from a Christian perspective, depending upon how a person meets their end. It's something that is dealt with in more detail in the 'to be or not to be' speech and the 'gravedigger' scene. 

Hope that helps!

Dlennie pats her head too hard, and starts to panic