Also remember Songkran is during April in Thailand. You may or may not want to plan on being in Bangkok during that time. Have you already made your flight reservations? Maybe a stopover in Tokyo would be an option for you. Although the cherry blossom window is small you may be able to catch it.

Also what about Singapore, Hong Kong or Siem Reap(Angkor Wat). With only four days you really only have time for a city stay. Trying to see even one place in Myanmar/Burma, which has limited flight options, seems to be a bit much.