So I never played the franchise before beyond just the demo for 4 (felt really clunky so I never played the full game), but 5 at least looks interesting, does one think I need to know the four others and play them to look into this one? or is it still too early to say?
I'd say you should play them all to fully understand the game. I played 3 before 1 and 2, and even if it's a prequel there were some concepts that I didn't get because I didn't play the previous games. There will be a lot of characters and relationships that are there to make fans say "gawd this is awesome", and that would make a starter in the mgs franchise just say "oh, cool", lol. Dunno if I'm explaining myself.
You should try them all... every single game is great, and 1 and 3 are just amazing... although I can't be objective cause it's my favourite franchise.