He's talked about in both past and present tense like others are saying here, so really it's hard to tell. Apparently Snake is going to be the main character of MGS5 (and he's seen with some new girl), but that really doesn't mean anything. They game could take place before 4, or even before 2 or 1. Heck could even take place before MG.
Or it could even take place in the six month he has left to live time period after MGS4. So yeah.
Here are the codec conversations about Solid Snake and Rose from early in the game.
I haven't gotten far into the game but these are the conversations most people seem to miss.
Probably unnecessary, but might as well point it out anyways as TVTropes takes the grammar tense used to indicate that Snake's alive:
Just because Raiden referred to Snake in the present tense doesn't mean Snake's alive. Don't forget, in Peace Walker, Big Boss referred to a guy he just jabbed with the stun rod in the past tense nearing the end of the prologue, even though it's obvious the guy survived (It had 2 million volts on the thing, which was stated explicitly in-game to only be powerful enough to knock someone out cold, not kill them).