The other guy seems to think that its 4-6 keys. the 6 key part is wrong, but I do agree its hard to get for 4 and under.

first off his links to support the raise:

http://www.tf2outpost.com/trade/9255894 - 4 keys offer

http://www.tf2outpost.com/trade/9241453 - S.F. Holy Mackerel (6 keys) offer

http://www.tf2outpost.com/trade/9244801 - 6 key offer in items

http://www.tf2outpost.com/trade/9244801 - Selling for 5 keys, many adds

http://www.tf2outpost.com/trade/9243197 - Sold one at 5 keys

http://www.tf2outpost.com/trade/6904069 - Sold one at 5 keys

http://www.tf2outpost.com/trade/9276191 - 4 key offer, rejected

http://www.tf2outpost.com/trade/9252204 - 4 key offer

http://www.tf2outpost.com/trade/3134734 - 4 keys offer

http://www.tf2outpost.com/trade/7703678 - White bill's for mantreads + holiday punch, and holiday punch is 4 keys atm, so the offer is about 6 keys

http://www.tf2outpost.com/trade/9259463 - Added for 5 keys

http://tf2tp.com/trade.php?trade=9243975 - Sold for 5 keys

http://www.tf2outpost.com/trade/9308113 - 6 keys offer in items

http://www.tf2outpost.com/trade/9221033 - Selling for a bill's, added

http://www.tf2outpost.com/trade/6646277 - Sold some at 5 keys

http://www.tf2outpost.com/trade/5568831 - 5 keys C/O

and now my links to support the cap at 5 keys.

http://www.tf2outpost.com/trade/9307720 SF banner (5 key item)

http://www.tf2outpost.com/trade/9344782 5 key 2 ref buyout

http://www.tf2outpost.com/trade/9352313 5 key buyout

http://www.tf2outpost.com/trade/7370745 5 key buyout

http://www.tf2outpost.com/trade/9347554 5 key buyout

http://www.tf2outpost.com/trade/9291196 5 key buyout

http://www.tf2outpost.com/trade/6904069 5 key buyout (3 in stock)

http://www.tf2outpost.com/trade/9236384 5 key buyout (8 in stock)

http://www.tf2outpost.com/trade/9339413 5 key buyout

http://www.tf2outpost.com/trade/9352101 5 key buyout

http://www.tf2outpost.com/trade/9259463 5 key buyout

http://www.tf2outpost.com/trade/9244801 5 key buyout

http://backpack.tf/classifieds/search/strange%20mantreads (4 keys, 5 keys, and 5.5)