The product directions on my cast iron skillet say not to go above 400 but I thought I had to use a higher temp? Help!

I had heard tales of cast iron cracking at very high temperatures if there were any flaws in the casting. However, I have a small cast iron barbecue from Lodge that handles direct contact with hardwood charcoal at hellacious temperatures with no problem- I don't think you have much to worry about.

Does your cast iron have an enamel or coating of some sort? These coatings can melt or crack above 400 F. If your skillet is simply iron, than it should be safe to go above 400 F.