Another Cockfighter Suggestion. No buyers for 4 keys. There's no demand on these things and many people offering only 2 keys on some trades or random items.

Some proof: (Will try to collect more)

http://www.tf2outpost.com/trade/9392401 (Asking for 4 keys)

http://www.tf2outpost.com/trade/9401674 (Asking for 4.5 keys)

http://www.tf2outpost.com/trade/9403760 (Asking for 4 keys)

http://www.tf2outpost.com/trade/9401996 (Asking for 3 keys and 3 refs)

http://www.tf2outpost.com/trade/9403624 (Asking for 4 keys)

http://www.tf2outpost.com/trade/9403392 (Asking for 4 keys and a ref)

http://www.tf2outpost.com/trade/9396564 (Asking for 4 keys or festive scatter + key)