Market and Classis are flooded with 4 keys each. Sellers go under the current.

http://backpack.tf/classifieds/search/444/11/0 14 for 4 keys 1 for 3.75 1 for 3.5

http://www.tf2outpost.com/trade/9545038 Selling for 3.5

http://www.tf2outpost.com/trade/9512010 ^ Same; 1 day old

http://www.tf2outpost.com/trade/9515361 ^ 1 day old

http://www.tf2outpost.com/trade/9538508 Selling for..2 D:

http://www.tf2outpost.com/trade/9503291 Buying for 3; got an offer of 3.5

http://www.tf2outpost.com/trade/9509059 B. for 2.5; has a new one in his Bp

http://www.tf2outpost.com/trade/9307207 Buying for 9 ref;got an add

http://www.tf2outpost.com/trade/9499029 Could buy some for 3 keys

http://www.tf2outpost.com/trade/9394999 Buying for 3. Got 4 added comments.

Unsold:

http://www.tf2outpost.com/trade/9419040 4 days old for 4 keys

http://www.tf2outpost.com/trade/9422756 Unsold for 4 keys; 4 days

http://www.tf2outpost.com/trade/9528738 Has alot for 4 keys; Unsold

http://www.tf2outpost.com/trade/9268985 ^ Same has 4.

http://www.tf2outpost.com/trade/9297245 1 week:best offer ~3.5 keys

Some very high/low sales still happen but those are rare exeptions. Quickbuyers have success when buying for 3, but offers of 3.5 also happen, therefore the range. Feel free to comment :D