The grocery store is always going to be cheaper. $4 is the low end of a single scoop in the bay area. And not for some hipster, plant-based organic ice cream either, this is at a regular parlor like Fenton's.
I’m not sure why you mention Fenton’s. I’m only aware of a single location in Oakland and I would describe it as fairly high-end. Perhaps not “hip,” but definitely very “retro” stylized. Very Instagramable. Long lines.