#16 would be post cat sensor...so that...not the #1 or the #15...again, on the assumption that the #4 sensor code you got was bank 2 sensor 2...can't imagine it'd be anything else.
Pricey bugger, but I know you can find them for under $100 around.
Also, first I'd make sure that the connections are good. Have you cleared the codes and the same one returned?
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