Your original link is a bit light on the specs. Where did you see the -4 temperature limit?
<cynic mode on>
Working on the principle that if it seems too good to be true, then there's a catch in it somewhere...
a) A bag described as a 500g down bag would normally contain 500g of down, to which the weight of the material has to be added. This would be somewhere in the 300 - 800g range depending on the materials used.

Down comes in various qualities, the cheaper of which can contain up to 20% of small curly feathers. There's about a 3:1 ratio between the weight of cheap down and the weight of the best down required to fill a specific volume.
c) There's various values for lowest temperature that can be quoted, even if just selecting from the current "official" set of values. There's the comfort limit, the lowest temperature at which you could expect to get a good sleep without feeling cold, and the extreme limit, which is the temperature below which you'd start to worry about freezing to death.
As I see it the possibilities are
1) The bag contains 500g of fairly cheap down, weighs in at 11-1200g total, and will probably keep you comfortable down to about +5 (May-September)
2) The bag weighs 500g total. In that case it's not got much down, and the -4 temperature will be where you worry about hypothermia
I bank on (1), and it will be similar to this
£50 bag from F&T
<cynic mode off>