The current price is outdated. Proof is below. A lot of unsold at 4.5 keys. Still possible, but a bit difficult to sell for 4 keys, sellers are going lower.

Sellers:

http://www.tf2outpost.com/trade/9189947 - backpack.tf high-end (4.5 keys), unsold, 1 week

http://www.tf2outpost.com/trade/9283783 - 4.5 keys, unsold, 1 week

http://www.tf2outpost.com/trade/9198482 - 4.5 keys, unsold, 1 week

http://www.tf2outpost.com/trade/9164127 - 4.5 keys, unsold, 1 week

http://www.tf2outpost.com/trade/9383986 - 4.5 keys, unsold, 5 days (with pyros killed attached)

http://www.tf2outpost.com/trade/9473382 - 4.5 keys, unsold, 1 day

http://www.tf2outpost.com/trade/9475210 - 4.25 keys, unsold, 1 day

http://www.tf2outpost.com/trade/9241688 - 4 keys, 1 week, unsold, one willing buyer (probably retracted)

http://www.tf2outpost.com/trade/9275245 - 4 keys, 1 week, unsold, got offered 3.5 keys

http://www.tf2outpost.com/trade/9335707 - 4 keys, unsold, 6 days

http://www.tf2outpost.com/trade/9469089 - 4 keys, unsold, 1 day (without bumps)

http://www.tf2outpost.com/trade/9480440 - 4 keys, 20 hours, no takers

http://www.tf2outpost.com/trade/9494762 - 4 keys, still new trade

http://www.tf2outpost.com/trade/9500936 - 4 keys, still new trade

http://www.tf2outpost.com/trade/9502966 - 3.5 keys, still new trade

http://www.tf2outpost.com/trade/9346901 - sold for a geniune Buck Turner All-Stars and a strange Sandman (~4.5 keys in items)

http://www.tf2outpost.com/trade/9422065 - sold for a clean Kringle + buildings destroyed part + 2 ref (~4 keys in items)

http://www.tf2outpost.com/trade/9401676 - sold for 3 keys

http://www.tf2outpost.com/trade/9487256 - sold for 3 keys

Buyers:

http://www.tf2outpost.com/trade/9215556 - bought for 3 keys + a silver sticky with buildings and robots destroyed attached (~4 keys mixed)