Koh Samui how much to get there and stay 4 days?

The best way for you is to take a bus from the Southern Bus Terminal in Bangkok to the pier in Surat Thani, then the ferry over to Koh Samui. Cost depends on what type bus. You can take a 24 or 32 seat bus. My best estimate is 900 baht give or take. The ferry is cheap, I always take my car and its no more then $10 for the car and two of use. I think the ferry is no more then 100 baht per person.