If another effect causes one of the returned permanents to become a creature and sets its power and toughness as it does so, that creature will have that power and toughness; it won't be 4/4. Notably, crewing a Vehicle does not set its power and toughness, so a Vehicle will remain a 4/4 creature if you crew it.

If a card in a player's graveyard has in its mana cost, that X is considered to be 0.

An Equipment that's also a creature can't be attached to anything. You can activate its equip ability, but it won't become attached.

An artifact creature or enchantment creature returned this way will be a 4/4 creature.

If X is 6 or more, the artifacts and/or enchantments enter the battlefield as 4/4 creatures. They don't enter as noncreature permanents and then become creatures.