#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?
I'm so depressed. No one wants to come see me in NYC: Sat, Oct. 8, 6 p.m. at Greenwich Village Comedy Club.
Mangofy your E46!