Why though? I know for sure you played the dragon age games which had 4 party members. Why disqualify this specific game solely based on this?
@Tuco's already said what I would say, which is that in a classless (or pseudo-class) system like DA and D:OS a party of 4 is fine. In a system that is the classic standard for being all about classes (and yes this is how I see D&D even with all the 5e changes; D&D is ALL about classes as far as I am concerned), and where that system provides so many classes (and then also subclasses, archetypes, and multiclassing), anything less than 6 for my party size is aggravatingly limiting. As I have been posting on the P:WotR forum, one of the things I love the most about party-based RPGs is assigning specific party roles to my party members (and again, please don't bother telling me that in 5e this is not needed because I don't care). I need in my party: a tank, a melee damage dealer, a ranged damage dealer, a healer, and a crowd-controller and buffer. The sixth spot, then, is for that eclectic companion who may not necessarily satisfy a particular party role but who brings something unique or special to the party. So, 6.