When I cast Munda, Ambush Leader and choose to look at the top four cards of my library, do I need to reveal all ally cards, or can I choose which ally cards I reveal and put the rest beneath my library?
You choose which ally cards you reveal. The text on the card says "Reveal any number of Ally cards from among them". "Any number" means you can choose to reveal 0, 1, 2, 3 etc Ally cards. And all the revealed cards are put on top of your library in any order.
Note that MTG never has instructions long the lines of "Reveal all Ally cards in your hand", because if you had 4 Ally cards but only revealed 3, your opponent couldn't have any way of knowing that you didn't reveal them all. MTG never requires "trust" in that way.