Mandalay and THEhotel skip 40s and 50s. I don't know the significance of "five" but every villa accommodation built by Steve Wynn lacks a unit numbered four or five. Wynn and Encore lack 40s as well as 13.

The reason Caesars, Sahara, and a few other hotels have curious floor numbering systems is so they can assign more unique room numbers to prevent confusion when signing charges to your room.