That sounds really unlucky.
If it reboots out of the box … contactsupport to get it replaced, or seeing that it would be the fourth phone already, money back?
If you happen to have fairphoneangels in your vicinity, they perhaps could have a look whether there’s something obvious that can be fixed immediately (like the battery sitting a little loose or similar), or they could try to swap modules to identify a hardware culprit to exchange before returning the whole phone.
Most of them don’t have this issue, else the forum would be really (like, really) flooded with this and Fairphone could be out of business by now I guess. But reports in this forum here prove time and again that indeed there is an unsatisfactory variation in build quality.
As reality would have it, most people content with their tech wouldn’t bother to report the fact that it just works. And if they did in large numbers, it would be kind of boring probably and it wouldn’t help people with problems either.
So, not all hope is lost if you still wanted to try again, even if I can understand you would think otherwise now.
I know it does not help your case, but since you asked, we even have a topic …