C/Os are only taken as far back as 2 months ago.

http://www.tf2outpost.com/trade/1003479

No B/O, C/O: 4 keys

http://www.tf2outpost.com/trade/3444076

B/O: 7 keys, C/O in comments: 4 keys 1 ref

Possible fake C/O of 11 keys

http://www.tf2outpost.com/trade/4831864

No B/O, C/O: 7 keys for naughty + nice crates

http://www.tf2outpost.com/trade/5432278

No B/O, C/O: Salvaged Crate #40 for naughty + summer crates

http://www.tf2outpost.com/trade/6478312

B/O: 4 keys, C/O: 6 keys for naughty + nice crates

http://www.tf2outpost.com/trade/2889082

No B/O, C/O: 7 keys for naughty + nice crates

http://www.tf2outpost.com/trade/7046774

No B/O, C/O: Crate #40 for naughty + summer crates

http://www.tf2outpost.com/trade/6226915

B/O: 7 keys, no C/O

http://www.tf2outpost.com/trade/6572468

No B/O, C/O: 4 keys 1 ref

http://www.tf2outpost.com/trade/7037967

No B/O, C/O: 6 keys

In conclusion, there are lots of 4 key offers on it, and some even higher, with one having a buyout of 4 keys. This, 4 keys is the best current price for the item.