Why does Phone storage get full with ambiguous "Other" storage that wont let me use more than one GB of the supposed four that i have

My HTC keeps running out of phone storage, there is supposed to be four GB of storage but it always gets full at one GB of app data and 3 gigs of "Other" data which isn't cache and I can't even see what it is, I've had this problem with previous HTC phones, I've done what I can to move as many apps to my SD card but most can be moved there. What is this Other, is it important data, and can i move it or get rid of it.

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