People don't use 4men grens because they can't bruteforce in the same way they can with other mainlines(something you can do better with 5 men grens).
Their inability to survive explosive spam is much less severe when you get to vet 3 with them. They were specifically buffed in that regard in the september balance update last year:
''
Grenadiers
We think Grenadiers are well-priced for their combat performance, but with only 4 models and reliance on being stationary to deal damage, it made them very susceptible to artillery and AOE attacks. Their veterancy has been adjusted to allow late game Grenadiers to resist explosives better, while retaining their current survivability against small-arms.
Veterancy 3 -23% Received Accuracy replaced by -20% damage reduction
''
So no, grens are fine balancewise. It's that other options just tend to be more popular because you can A + M1 more effectively.
Yeah, I know the existance of that new damage reduction since a few patches ago.
But first of all, you have to reach vet3 with grens. You have to survive the LV stage, specially T70.
This is another example of retarted parabolic buff by Relic, when all that is needed is add a 5th men from stock, or maybe an upgrade like Cons or Sections.