In 5E, Bard are full caster. I would agree than in most rpg, support classes tend to shine with larger party. However, that's not what bard are in 5E.They can fulfill almost any roll (from healer to CCer, etc.) and be really good at it as long as you build them appropriately.
I'm not knocking bards, I've played a couple, I've played with several others, they are a fun class they are a cool class, but they struggle to fill any of the staple roles (CC isn't a role on it's own) and with only a 4 man party it's rough to find a spot for them. They aren't going to out rogue a rogue for trap finding and damage, they aren't going to out wizard a wizard as the arcane caster/ranged damage dealer, they aren't going to out tank a fighter as a melee beatstick. They can be just about as good (only lacking revivify) of a healer as a non-life domain cleric or druid so they got that going for them.
Having said all that, this topic was initially raised about CHA based characters in Early Access. The Warlock was the CHA based character they chose over bard, they were the most unique.
Now, bards are coming, maybe later in early access, maybe in the full game, and I will definitely be playing one weather they expand the party size or not. If they don't I will take my bard as a healer who can CC, and if they do increase the party size I'll have all kinds of options for party balance to play around with and many types of bards become valid.