I wasn't speaking of a 4k TV, but still, this doesn't check out. A single 2160p framebuffer is 8MPix, or 32MiB. Not counting the original FB size, the extra 1.5GiB are enough for 48 whole framebuffers. You don't need that much image data all at once, the number is ridiculous. No, I believe it's just that the code became that much less efficient.
Think of each app and all the texture content that needs to be loaded. App textures get 4x as big, all things being equal. You see a 4x change in ram across those devices.
Still the TV does fine with 2GB. Doesn't seem fair to complain.