There are, indeed, four boxing stadiums in central Chiang Mai. Of these the main and "official" Muay Thai stadium is Thapae Stadium, which is where all the best fights in Chiang Mai happen. If you are going to only one of them, go to Thapae Stadium. There are fights every night apart from Sundays - or at lest there were before the pandemic. As you might guess, it is located near the Thapae Gate.

The other three venues are in the Night Market area. These are Loi Kroh Stadium, the one in the night market building opposite the Hard Rock / Red Lion pub (you will need to go up to the first floor); and the one in the Kalare Night Bazaar building. Of these Night Market options, the Loi Kroh stadium is the best. All these stadia are within walking distance of each other. Central Chiang Mai is small.

No idea where you are staying, but if you come from the direction of the Night Market / Le Meridien hotel, the entrance to Loi Kroh stadium will be on your left. Calling it a stadium is a bit of an exaggeration though. It is a raised boxing ring within a bar area, surrounded by bars. It isn't visible from the main street but you can't miss it as there will always be a ticket desk and touts out on the main street at the entrance to the passageway leading to the "stadium". The fights in Loi Kroh and the other two Night Market venues attract a lot of juniour Thai fighters and foreigners who are studying Muay Thai in the many Muay Thai schools in Chiang Mai. Don't expect to see the kind of fights that you will see at Thapae Stadium though. Tickets at the Night Market are a standard 600 baht with free entry for kids accompanied by adults.
Prices at Thapae Stadium very by ticket type, from 600 baht - 1,500 baht.

My guess is that having been for one fight you will want to go back for more. Thapae Stadium is the best choice, followed by Loi Kroh.

Incidentally, for the causal Muay Thai fan or foreigner just curious to watch the fights, Chiang Mai is probably the best place to watch one. The Bangkok stadiums attract the very best fighters, but prices are higher and the size of the stadia mean that you will be further away from the action. In Chiang Mai, you will be closer to the action and will often have the opportunity to approach the fighters after the match and to have your picture taken with them etc. Tips are always appreciated, of course.

Have fun in Chiang Mai.