The Kellison J4 is a rare fiberglass-bodied kit car that a few fans hope shows up as a classic sports pick in GTA 6. The real car is a low, light 1950s and 1960s special built for looks and speed on a home-built chassis. Rockstar has not confirmed it, and there is no in-game name for it.
There is no in-game name or unlock for this, so treat it as a niche fan hope. The interest comes from how rare and distinctive the real kit car is. If something like it appears, expect a light, low-slung sports car with lively handling that depends heavily on whatever engine sits underneath.
The community companion for Grand Theft Auto VI. Database, maps, guides and tools for Vice City and the state of Leonida.