Thanks for the report. I was able to reproduce this internally so I’ve created JIRA UE-38143. Our developers will be investigating further. You can follow that link to the Public Bug Tracker to keep track of it’s status.
If you make a third custom variable with 3 defaults one, the one with the ‘3’ at the end will cause complation errors. I’m assuming if you make a fourth custom event with 4 variables, one of them cant be newparam4.