You check if you control a creature with power 4 or greater as Icy Blast resolves. It doesn’t matter if you control such a creature during any of the relevant untap steps.

Icy Blast can target a creature that’s already tapped. It still won’t untap during its controller’s next untap step.

Icy Blast tracks the creature, but not its controller. If the creature changes controllers before its first controller’s next untap step has come around, then it won’t untap during its new controller’s next untap step.

Some ferocious abilities that appear on instants and sorceries use the word “instead.” These spells have an upgraded effect if you control a creature with power 4 or greater as they resolve. For these, you only get the upgraded effect, not both effects.

Ferocious abilities of instants and sorceries that don’t use the word “instead” will provide an additional effect if you control a creature with power 4 or greater as they resolve.