I would be at the airport around 4am. I prefer to have more time than rushing and worrying. A few hours at the airport will be pleasant and will past real quick. There are restaurants, shops, toilets and aircon. A good place to relax actually.
Train from Nakhon Sawan to Bang Sue Train Station. Take MRT Blue Line to Phetchaburi MRT. Change to Airport Rail Link to Suvarnabhumi.
Or Train to Don Muang and take a free shuttle bus to Suvarnabhumi which takes about 1 hour. You need to show the staff at inside counter your ticket and theyโll give you a sticker before getting on the bus.