According to FastLaneGaming, there's a P1 GTR coming! (it is NOT the owen's edition because if you look at the back it has that big carbon piece like the real one) :
Forza horizon 4
McLaren P1 going supersonic : r/forza
Forza Horizon 4 recibe dos nuevos McLaren | Eurogamer.es
Las preparaciones más impresionantes del 'Forza Motorsport' -- Tecnología -- Autobild.es
P1, forza horizon 4, mclaren p1, motores, Fondo de pantalla de teléfono HD | Peakpx
Forza Wiki - Mclaren P1 Forza Horizon 4, HD Png Download , Transparent Png Image - PNGitem