404 redirect or disconnect...

[AJ Weber]

I have a custom site where valid users will always login and have a 
valid session (and a cookie or headers to represent it).

I would like to do the following, but not sure how to start...can 
someone just point me in the right direction?

IF user has a valid session (I need to check if the header/cookie is in 
the http request), and a 404 is appropriate, re-direct to a specific 
location (or a "real" 404 page).

ELSE (the user does not have a valid session for my 
website/application), return 444.

Really, I need to understand how to do the if/then and detect the cookie 
or header to indicate that the user is "valid".

Thanks in advance,
AJ