Why does my battery go dead every 3-4 days when car is parked?

You need to have the vehicle checked for a parasytic draw.

You have an open curcuit somewhere constantly drawing current obviously not much so something low amp is shorting out, unfortunately it could be any thing, start unplugging things one at atime ( process of elimmination) like start with the fuse panel or go to an auto wiring specialist & they will have the proper equip. to trace the short