April is the summer in Thailand ,the hottest month that is very hot and humid. Don't be afraid of that heat, it around 36-39 degree celcious.If you will be heading to beaches, you will get some nice sea wind.
During 10-15 of April, we have a festival called "SongKran" when Thais pour water and give nice wishes to each other on Thai's traditional new year April 13th. This is the wisedom of teh ancient Thai, using water to release April hot.
If your trip is during such the period, you can join the festival which take place everywhere in Thailand. Many foreigners come every year just to join the festival. The most popular place in Bngkok is on Koasarn Rd. where so many tourists jam the crowd and play water guns a long the road. SongKran in ChiengMai is also very wellknown with many cute smiling Thai girls in wet dress..
Anyway, if you plan to go to beaches, you can have Songkran everywhere too. It's a very nice festival I recommend you to try.
Have a great time even in April.