I'll just try.

Unsolds 4 keys:

http://www.tf2outpost.com/trade/26138479 - Restocker (haven't sold any in 10 days.)

http://www.tf2outpost.com/trade/26386664 (young)

http://www.tf2outpost.com/trade/26363635

http://www.tf2outpost.com/trade/26384211 (young)

http://www.tf2outpost.com/trade/26078798 - restocker (not sure, but doesn't seem to have sold any the last 4 days.)

http://www.tf2outpost.com/trade/26318039 - 1 week

4 keys does not sell that easy i'm pretty sure we can conclude.

Unsolds 3.5 keys:

http://www.tf2outpost.com/trade/26367412 - 2 days

http://www.tf2outpost.com/trade/26368372 - 2 days (poorly bumped)

hmmmm...

Solds at 3.5 keys, and lower

http://www.tf2outpost.com/trade/26345602 - sold, not even bumped once.

http://www.tf2outpost.com/trade/25982680 - sold 5 at 3.31 keys each

http://www.tf2outpost.com/trade/25845519 - 3.63 keys

http://www.tf2outpost.com/trade/25810100 - 3.50 keys (restocker)

http://www.tf2outpost.com/trade/26321593 - 3.45 keys (did take him around 1 week to sell)

http://www.tf2outpost.com/trade/26342309 - sold 5 at 3.45 each, trades been up for 5 days.

bp.tf classifields ( lots of unsolds for way under 3.50 keys:

m.imgur.com/kjzpfHn

Seller at 3.05 keys (quickseller)

Seller at 3.11 keys (most likely quicksale aswell

Seller at 3.16 keys (idk)

Seller at 3.21 keys (idk)

Seller at 3.26 keys (Bot + restocker.)

Conclusion

4 keys simply sells way to slow, and basicly does not really sell.

3.50 keys sells not quick, but still sells fast enough to include in the range.

2 bot restockers selling at 3.25 each on classifields.

A lot of sales in between 3.50 and 3.25 as shown above.

Therefore my suggested value is 3.25 keys - 3.50 keys.

I used a lot of time on this, hope i didn't mess it up.