Wrangle can target any creature with power 4 or less, even one that you already control or that is already untapped.

If the target creature's power becomes 5 or greater as you gain control of it (perhaps because an enchantment gives creatures you control +1/+1), Wrangle will continue to resolve as normal. The creature will still be untapped and gain haste until end of turn, and you'll still control it until end of turn.