I see what you mean but by that standard almost any shooter with a supernatural enemy is a survival horror, you have to survive and your enemies are of the horror nature.
Take Half-Life 2, there are alien enemies and you have to survive, yet it is not classes survival horror.
The thing I guess I'm saying is
L4D is looking more like your average zombie shooter + cooperative...
If certain things do not characterise a genre then is it really a genre?