Okay, a couple of questions to narrow down when and how this problem happens might prove useful. Did this error message only show in the Charactermancer window, or was it overriding the whole character sheet page or the entire game page? If you exited the situation & tried again, did this error occur again? Was this character originally created before this feature was launched? If you create a new character and level it to 4th lvl and try to add a feat, does this error happen again? If it repeats, are you able to avoid the bug when creating a new lvl. 4 Chracter in another campaign? I had no trouble with creating a new character & adding feat at 4th lvl, but haven't tested levelling an old character. Does your character have a feat from 1st lvl, and if so, does your game have only access to the SRD compendium, or more? An unlikely, but possible, edge-case would be that in case you only have access to the one Feat shown in the SRD and you have already given it to the character, the Feat-tab(if poorly implemented) could bug out if it has no unused Feats to display. Do you use a browser other than Chrome or Firefox? Unlikely to be a browser problem, but better be on the safe sid and use the recommended browsers.