If budget is not a problem, I suggest you book one of the low cost airlines like Nokair or AirAsia so you will have more time to spend in Chiang Mai. They are both reliable, but delays could be an issue .. only if you are really unlucky. Nokair is based in Bangkok old airport (Donmuang) and Airasia in the new Suvarnabhumi airport. I have used both and they are fine.

Otherwise, get on a sleeper train in the evening and arrive in Chiang Mai early morning. The train ride is pretty comfortable but takes you around 12 hours to get there (from my experience). So if you only have 3-4 days to spend I'd prefer you to fly. Right now the air fares are reasonable and if you check out the website of Nokair, for example, there are plenty of time slot available for you to pick with different fares. On average, you could be paying around 3300 baht for a return flight per person.

Accommodation .. depends what style of traveler you are and what is your budget. There are numerous to choose from!