I think it's pushing it a bit. If you have to take one out of the equation, you should make it Koh Lanta, if you're really into the rock formations, or maybe cut Bangkok down to 1 night. It's not that great. I'm a city person myself, but Bangkok gets boring pretty quickly. You can see and experience most of the stuff in 1 day.

Phi Phi Leh and Phang Nga Bay (James Bond Island) both have breathtaking rock formations.

Yes, Lanta to Krabi is easier than Lanta to Phuket, because the Lanta to Phuket ferry has to stop at Phi Phi on the way back.