I found a half decent work around for this when I was playing the mage, but it's really for if you're using any shapechanging spell as a last resort. First, determine what spell slots you lose. When preparing your spells, put all of your buff spells or "use me first" spells in the slots that would be disappearing. Then...ya know... use 'em first. If you're a sorcerer, polymorph on your last level four opening. If you're a wizard, put polymorph in the last level four slot. You still get to shapechange and also get to use up all your daily allowance of hellfire. You say you're losing all your level four slots, but if they've already been cast as extended or empowered buffs, no real loss. Also, if one of the buffs is an extended or empowered fox's cunning (or eagle's splendor, owl's wisdom) your spellcasting stat doesn't take so much of a hit and you might not lose as many spell slots. And you're a much beefier spider. Or umberhulk...whatever. But not zombies, cause they're farkin creepy.
-Pox