Proof:

Last Suggestion (Credit to CheezeMunchy):

http://www.tf2outpost.com/trade/17064539 - 4 keys, sold

http://www.tf2outpost.com/trade/17011701 - 4 keys, sold

http://www.tf2outpost.com/trade/17064243 - 4 keys, sold

My Proof:

Quicksales below 4 keys:

http://www.tf2outpost.com/trade/17250621 - 3.6, less than 7 hours

http://www.tf2outpost.com/trade/17249490 - 3.76, 3 hours

Sales at 4 keys:

http://imgur.com/dJ7mv26 - 2 sales

http://www.tf2outpost.com/trade/16939397 - Sold 1, has a willing buyer

http://www.tf2outpost.com/trade/17207329 - Sold 2 in less than 2 days

http://www.tf2outpost.com/trade/17248415 - Less than 12 hours

http://www.tf2outpost.com/trade/17235450 - 8 hours

Less than 2 pages on outpost

There are a few sellers below 4 but it goes really fast, and as you can see, even 4 goes pretty fast

Considering there are almost no sellers, below 4 seems rather fast and an outlier compared to 4