Try and keep to the coast as much as possible. It is longer but much nicer. When you get to Chumporn turn right and go over to Rayong - this is an excellent route to ride great rode and scenery
Yes road 4 from Chumphon to Ranong is not a dual carriageway and is a good scenic route, if you spend a couple of nights in Ranong there are some lovely loops across the mountains that you will find in my trip reports