Maybe you have them too close to the flange? I believe 2-4 inches, but closer to 4 inches will make them last longer.I had two go bad in just under 100hrs.
Have you thought about swapping probes from a good cylinder into the bad cylinder, like put the #2 probe (wire and probe) into the #4 cylinder, so you will be reading the temp of #4 with the #2 bar on the read out. If you still see the temp spike then you know for sure it is not the electronics.
I just replaced my left probe at 260 hrs time in service. It was spiking high. I tried re terminating the probe to no avail and swapped inputs to verify the failure. The Dynon replacement was also bad, but its symptom was that it peaked at about 500 F. Three was the charm. The second replacement works as advertised.