Okay so I set my mavic pro batteries to discharge after 4 days, however, since I've had a gimbal error and had to send it back it has been more than 10 days and my batteries are still full or < 75/80%. Why is this happening, shouldn't they start discharging without me having to do anything?

It's quite slow, furthermore if you check them it seems to reset the countdown so wait 4 day minimum!
The setting you selected might have been reverted to default in battery firmware update but they'd still go down after 10 days.
Discharge takes place over 2 or 3 days

As 4wd already mentioned, the countdown timer gets reset when you push the battery's button and can take a few days once it starts.

Didn't know about the counter reset, thanks. They mostly have been in my bag is I cant use them right so I'll give it a few more days and then re-check