You don’t choose a power and toughness for Master of Winds as its ability triggers. Rather, as it resolves, you choose whether to have Master of Winds become 4/1, become 1/4, or not change its power and toughness.

The ability of Master of Winds overwrites any previous effects that set its power and/or toughness to specific values. Other effects that set these characteristics to specific values that start to apply after the ability resolves will overwrite that part of the effect.

Effects that modify the power or toughness of Master of Winds without setting it will apply to its new base power and toughness no matter when they started to take effect. The same is true for counters that change its power and toughness.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to Master of Winds may become lethal if you change its base toughness during that turn.

An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger, but after targets have been chosen for that spell (if it has any targets). The ability resolves even if that spell is countered.

A Wizard spell is one with the creature type Wizard. Spells that are Wizard-themed (such as Relic Amulet) aren’t Wizard spells.