Another very good resource for bike travel is

http://wikitravel.org

with plenty of info about hotels, hostels, sightseeing, restaurants. They have the following to say about the climate in April...

Indochina (north/central Thailand, Laos, Cambodia, Vietnam, Myanmar), the seasons can be broken down into hot, wet and dry, with the relatively cool dry season from November to February or so being the most popular with tourists. The scorching hot season that follows can see temperatures climb above 40°C in April, cooling down as the rains start around July.