I feel like this is an even trickier task than porting 2 & 3 too. MGS4’s reliance on the weird architecture of the PS3 means you need a powerful CPU to emulate it. Assuming then that it’ll be a port, I imagine the process of porting it would be particularly complex. And seeing how poorly they did with the HD collection it worries me.
I guess there aren’t many older games because it’s a list of games that are available on current hardware. Those older games that do appear are there because they’ve been ported/emulated.