I received this iPhone 4 from work, and they don't have access to the Mac used to supervise it anymore. According to this Apple support article, device supervision can be removed by using "Erase All Content and Settings". Unfortunately, I just found out that the device was jailbroken so "Erase All Content and Settings" will put it on a bootloop.
I can't seem to put the device on DFU mode; is this because it's supervised? Is there anything I can do to unsupervise this device?
EDIT: I finally managed to put it to DFU mode! Took a few tries, and had to exit/relaunched my iTunes a few times before it showed up as a device there. As @grgarside mentioned below, recovery mode should have worked too, but somehow my iTunes kept either not recognizing the device when on recovery mode, or it says that the device is supervised by another computer so it can't be used with mine.