Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Kuala Lumpur to Sakon Nakhon as you normally would.
You can imagine temperatures to be in the 35 °C range in April. The weather in Sakon Nakhon is generally hotter than Kuala Lumpur's. As flight availability is subject to become more limited as peak season approaches it is ideal to book in a timely manner.