Triangulation with 4 points

Question

I have a square room and in each corner there is a microphone that pics up noises. I need to find the position in space of this sound by using the time the sound needs to reach each microphone. I know how to do it with 3 mics, but not with 4. Als because with 3 there is only one solution, but eith 4 the values will not be perfect due to tolerances, reaction times, mic positioning and so on. How can the position be approximated by averaging the positions? The only idea i have is to calculate it 4 times by using 3 mics on every calculation and then pick the average position of the 4 results. But i'd guess that there is a better solution for this..