Wait, is it 2020 yet?
In the past, we’ve indicated 2020 as a likely introduction date for Raspberry Pi 4. We budgeted time for four silicon revisions of BCM2711 (A0, B0, C0, and C1); in comparison, we ship BCM2835C2 (the fifth revision of that design) on Raspberry Pi 1 and Zero.
Fortunately, 2711B0 has turned out to be production-ready, which has taken roughly 9–12 months out of the schedule.
https://www.raspberrypi.org/blog/raspberry-pi-4-on-sale-now-from-35/