Does "Destroy all creatures with power 4 or greater" also affect my creatures? I have a planeswalker card that has this ability; will it also kill my creatures? Also, this only affects creatures on the battlefield, not in players' hands, right?
It will destroy your creatures as well. Just read the card literally. There's nothing there to imply that your creatures would be excluded. As for players' hands, the word "creature" by default means "a creature card or token on the battlefield." You cannot have a "creature" in your hand, you can only have a "creature card" in your hand. Finally, note that "destroy" would have no meaning when used on a card in your hand, it would have to say "discard" to remove a card from your hand.