This has the supertype world. When a world permanent enters, any world permanents that were already on the battlefield are put into their owners' graveyards. This is a state-based action called the "world rule." The new world permanent stays on the battlefield. If two world permanents enter at the same time, they're both put into their owners' graveyards.

If the player whose turn it is has four or more cards in their hand as Storm World's ability resolves, the ability just won't do anything that turn.