The purpose of iPhone restrictions is to lock certain functions of the device. They are also referred to as Parental Controls, capable of hiding songs with explicit lyrics, preventing YouTube from running, etc.
Without the four-digit passcode restrictions can't be changed. If you have forgotten your restrictions passcode you must restore your iPhone using iTunes to reset the passcode. When asked, do not restore the iPhone from backup, otherwise the old passcode you don't know will be enabled.