Current price of 4 keys is 6 days old, and there are numerous trades selling at this price and lower.

Selling at 4 keys:

http://www.tf2outpost.com/trade/11173937 (3 days unsold)

http://www.tf2outpost.com/trade/11193274 (3 days unsold)

http://www.tf2outpost.com/trade/11220763 (2 days unsold)

http://www.tf2outpost.com/trade/11268311

http://www.tf2outpost.com/trade/11276545

http://www.tf2outpost.com/trade/11219181

http://www.tf2outpost.com/trade/11218843

http://www.tf2outpost.com/trade/11268637

http://www.tf2outpost.com/trade/11241163

http://www.tf2outpost.com/trade/11279459

http://www.tf2outpost.com/trade/11276393

Selling under 4 keys:

http://www.tf2outpost.com/trade/11193040 - 3 keys 4 ref, not yet sold after 3 days

http://www.tf2outpost.com/trade/11254760 - 3 keys, 3 refined

http://www.tf2outpost.com/trade/11282331 - 3.5 keys

http://www.tf2outpost.com/trade/11282045 - 3.5 keys

http://www.tf2outpost.com/trade/11222089 - 3 keys 4 ref

http://www.tf2outpost.com/trade/11226184 - 3 keys 2 ref

http://backpack.tf/classifieds/search/invis At time of posting, 2 for sale at 3 keys, with many unsold at 4 keys.

Screenshot proof of classified listings: http://i.imgur.com/WN6AXwA.png

:)