I vaguely remember them saying there'd be a hardcore mode, but I think it's reasonable to expect it. It was widely popular in FO3 as a mod to the point of it being obligatory in FONV.
If you enjoy these games as immersive experiences it's an easy improvement to make. However, it only works as a gameplay mechanic when resources are scarce and the player is forced to think and make decisions about his hunger/thirst/radiation. FONV did not really attempt resource scarcity and modding resource scarcity is pretty hard given the massive number of ways to get resources, generate wealth and trade that wealth for resources. FO3 was actually better in that respect because it was generally bleak and less resource heavy.
With Rust, DayZ and other zombie survival sandbox games having become pretty popular it's possible that they'll reuse those ideas and make a scarce game but I don't think they will focus on that enough to do it properly out of the box. So from a gameplay standpoint whether there is a hardcore mode is basically irrelevant: FO4 probably won't work well as a survival game until it's heavily modified.