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4 of 4 added 44 characters in body

JavaScript (ES6), 31 29 28 22 bytes

s=>(15-`0x${s}`%15)%15

Port of @xnor's Python answer, except that JavaScript only has a remainder operator rather than a modulo operator, so I can't do it in a single step. Edit: Saved 6 bytes thanks to @Arnauld.