You can go along the Gulf coast from Phetchaburi (south of BKK) all the way to Surat Thani. It is quite easy to avoid Highway 4 by following small roads (in very good condition) between the highway and the coast - doable without a map, but you will need to speak a bit of Thai. The road is very quiet and very pleasant with some nice landscapes. It's pretty flat.
Then you can cross to the East coast to Takua pa. Very nice road, also quiet, a bit hilly and you go through the jungle (nice national park). You can then go along the coast to Phuket. The road gets a little busier but it's not bad. Nothing exceptional on this stretch. The road along the coast to Phanga and to Krabi is the most beautiful one I biked in Thailand (a couple of very nice national parks, and again, it is very quiet if you stay off the main road).
From Phuket you can go to Ko Phi Phi by boat (a touristy but beautiful island), then you can get a boat to Krabi. If you are looking for a peaceful, non-touristy and beautiful island, go to Ko Yao; you get there by boat from the northwestern part of Phuket Island. When I was there, the school would give you free lodging in a bungalow if you accept to talk to their English class. Beautiful deserted beaches (Thai people don't really like beaches like we do in the West and there are no tourists, so the whole paradise is for you). I attached a quick map with the itinerary.
Then you can take route 415 back to Surat Thani (nothing overly exciting but it is also quiet).
I hope this helps. Enjoy your journey, Thailand is truly an amazing country for bicycle touring.