What are some major themes in Hamlet throughout act 4?
Act 4 moves pretty quickly. Hamlet has just impailed Polonious (I was waiting for that!) and the theme of death and murder begins to take on greater significance. The theme of bravery and courage or rather Hamlet's lack of it comes into sharper focus. Hamlet pulls off another of his famous soliloquies. This one reminds us of the player getting all teary eyed over Hecuba. Only Hamlet woud be impressed by a dude crying over a story about an old lady and her dead husband. So, Hamlet goes off on himself with the usual "I'm such a coward" speech except this time it looks like he means business. The theme of masculinity or hamlet's sense of it can be seen as well,
“Rightly to be great / Is not to stir without great argument, / But greatly to find quarrel in a straw / When honor’s at the stake.”
We can see that Fortinbras has become something of a foil for Hamlet; Hamlet just hopes that if he can lift a lot of weights, he too can be a jock like Fortinbras.
There is the theme of the female subserviance and hence collateral damage, in Ophelia, that usually ends up in tragedies. Ophelia has gone "loony tunes" mostly as a result of patriarchal manipulation.