Taranika's triggered ability isn't optional. If your only other creatures are bigger than 4/4, you'll have to shrink one for the turn.

Any effects that modify the target creature's power and/or toughness without setting them to a specific number or value will apply after its base power and toughness are set, regardless of the order in which those effects were created. The same is true of counters that modify its power and toughness.

Untapping an attacking creature doesn't remove it from combat.

Taranika's triggered ability can target a creature that's already untapped. It still becomes 4/4 and gains indestructible.