4th weapon slot glitch

[RBmayhem]

Hello, ive reached level 50, completed the game, but my 4th weapon slot is still locked, ive gone through every part of every map done all side quests found all locations, but still no 4th weapon slot, im guessing it glitched when i was playing with a friend, does anyone know a work around or if it will unlock in true vault hunter mode, or a hack to fix this issue, the old games are easy to hack away problems like this but theres nothing out there yet

[mimizukari]

play through TVHM and DO NOT SKIP QUESTS THAT YOU COMPLETED IN OTHER PEOPLE'S LOBBIES.

the fourth weapon slot is unlocked after Beneath the Meridian.

[Nite69]

if its locked then you didn't complete it solo, only the host of the coop will progress