What it says on the tin, most of the cultures cap out at 3 mounted units if you count optional cav and one of those 3 is a scout. Generally speaking, this is a reasonable cap because mount traits will apply to tome units too, meaning you have options for a mount focused build. HOWEVER, I propose Primal gets one extra optional cavalry unit as a way to buff the culture as well as fit it's theme a bit more. As to which unit gets the mount trait I don't particularly care, I have a preference for Ancestral Warden but there is an argument to be made for each unit so I'm not gonna die on that hill.
I will say that Feudal is another fit for having 4 cavalry but again, I'm not dying on that hill. I simply think having an extra cavalry on the Primal culture would fit it's theme and give it a small buff.