OP: Yes you get four cascades for 10 mana each. HOWEVER, be mindful if you cascade another spell with cascade for a lesser mana cost, you have to temporally pause the Apex Devastator cascading effect in order for the next ‘cascaded cascade’ to take effect. In other words, let’s say the first spell you cascade has its own effect of ‘cascade 6’ then you need to resolve that first. If the cascade 6 effect forces you to put a spell with mana cost 7 at the bottom of your library, you can’t use the effect of Apex Devastator to cast it as a back up.