They don't need a fifth man. That said, I think their veterancy bonuses should be on-par with Allied infantry units. Riflemen have much better bonuses right now, and I'm not sure that should be the case.
The very moment grens utilities and weapon upgrades will be locked behind optional side techs.
Also, before the changes, rifles were most expensive mainline infantry with worst scaling in game by far, they were only buffed to actually be on pair with other units for cost effectiveness in late game as they couldn't keep up-they were fine early game, but were just an attrition grinder in late.