This is a counter-suggestion to the current '3.5 keys' flat suggestion. A range is more appropriate, I think.

Credit to |¢ĸ| Saint Jimmy and andrew for most of the proof.

unsold 4 keys:

http://www.tf2outpost.com/trade/15527425

http://www.tf2outpost.com/trade/15488264

http://www.tf2outpost.com/trade/15527351

http://www.tf2outpost.com/trade/15511288

http://www.tf2outpost.com/trade/15455784

http://www.tf2outpost.com/trade/15527777 also included 3 haunted demo items. (still unsold)

http://www.tf2outpost.com/trade/15459116

http://www.tf2outpost.com/trade/15470982

http://www.tf2outpost.com/trade/15455117

http://www.tf2outpost.com/trade/15444083

http://www.tf2outpost.com/trade/15507485 asking 5keys lol (unsold)

http://www.tf2outpost.com/trade/15501667

http://www.tf2outpost.com/trade/15503112

http://www.tf2outpost.com/trade/15428792

http://www.tf2outpost.com/trade/15485487

http://www.tf2outpost.com/trade/15523330

http://www.tf2outpost.com/trade/15522938

http://www.tf2outpost.com/trade/15520518

http://www.tf2outpost.com/trade/15503136

http://www.tf2outpost.com/trade/15114519 rejected a strange pomson thinks its low

http://www.tf2outpost.com/trade/15440234 asking 4keys 1ref (unsold)

http://www.tf2outpost.com/trade/15520949

http://www.tf2outpost.com/trade/15498342

http://www.tf2outpost.com/trade/15520131

http://www.tf2outpost.com/trade/15486635

http://www.tf2outpost.com/trade/15349368 asking 4.5 (unsold)

http://www.tf2outpost.com/trade/15441941 asking 4.5 (unsold)

http://www.tf2outpost.com/trade/15496060

http://www.tf2outpost.com/trade/15511771

http://www.tf2outpost.com/trade/15432390 looking for ofers (nothing so far)

http://www.tf2outpost.com/trade/15381527

http://www.tf2outpost.com/trade/15507631

http://www.tf2outpost.com/trade/15467082

http://www.tf2outpost.com/trade/15503391 asking for keys says he will add halloween items (still unsold)

http://www.tf2outpost.com/trade/15492752 asking 5keys lol (unsold)

http://www.tf2outpost.com/trade/15428385

unsold 3+ 3 ref:

http://www.tf2outpost.com/trade/15395120

http://www.tf2outpost.com/trade/15527903

http://www.tf2outpost.com/trade/15490342

http://www.tf2outpost.com/trade/15487658

http://www.tf2outpost.com/trade/15504495

Others below 4 keys

http://www.tf2outpost.com/trade/15508871 - 3 keys + 3 ref

http://www.tf2outpost.com/trade/15518900 - 3 keys + 3 ref

http://www.tf2outpost.com/trade/15503788 - 3 keys + 3.33

http://www.tf2outpost.com/trade/15476370 - 3keys + 4 ref

http://www.tf2outpost.com/trade/15512878 - 3 keys + 4 ref

http://www.tf2outpost.com/trade/15513032 - 3 keys + 4 ref

http://www.tf2outpost.com/trade/15525197 - 3 keys + 4 ref

http://www.tf2outpost.com/trade/15524835 - 3 keys + 5 ref

http://www.tf2outpost.com/trade/15504302 - 3.9 keys

not to mention several of them in classified for under 4keys (3.6-3.5)

http://backpack.tf/classifieds/search/id/521e11874f96f4526c8b4567

SOLD recently for 4 keys

http://www.tf2outpost.com/trade/15433775

http://s949.photobucket.com/user/tricross/media/739d748e8e4bd2c875a2085b390e7190_zpsf178efae.jpg.html

http://www.tf2outpost.com/trade/15519822

http://www.tf2outpost.com/trade/15518438

http://www.tf2outpost.com/trade/15509401

http://www.tf2outpost.com/trade/15506976

http://www.tf2outpost.com/trade/15504466