Bangkok - safari world - under ocean world - madame tussauds ( you can skip it if not interested in wax meausem things) - Grand Palace, wat Pho - some shopping malls if interested - Siam Paragon, Siam discovery, Terminal 21 etc. Pattaya - coral island - alcazar show in evening - walking street in night
For 4 days.. I would suggest 1 day in Bangkok if you must.. Wat pho is where most attractions are.
If you are looking for more natural places and relaxing - do visit Phuket, Krabi, phi phi.. plan for a stay in Phi Phi and enjoy the night life on the beach with fire shows and pool party.
You can go for packages offering boat tours with snorkeling, plan for scuba (need half day for it). Krabi tour have hot spring if you want to try. Phuket is also a good place to visit.. Massages are cheapest here than Bangkok and krabi.
Ko Samui is also a pretty island.
For more hilly areas and awesome temples- check out Chiang Mai. Don't worry about where and how to go once you reach there.. Just book hotel in the city and there will be lots of vendors providing day tours.