Y'shtola's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack.

Y'shtola's second ability checks at the moment it would trigger to see if a player lost 4 or more life this turn. If none did, the ability won't trigger at all. Once an end step begins, it's too late to cause a player to lose life in order to cause this ability to trigger.

For spells on the stack with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.