They probably wanted to fix the confusion about what type of core was fitted into each processor, as the 3000-series desktop processors contain Zen 2 for the parts without iGPU, but use Zen 1+ for the parts with iGPU. 4000-series currently is only parts with iGPU and Zen 2, so they have comparable performance to the 3000-series desktop processors without iGPU.
So, I expect that iGPU-enabled parts which contain Zen 3 will also be branded as 5000-series CPUs.
They skipped 4000 because of their mobile/APU series, which always used +1000 in the model number:
- Zen 2 desktop: 3000
- Zen 2 mobile/APU: 4000
This way, Zen 3 will all be on 5000
Number 4 is considered an omen in china. Probably changed it due to marketing reasons.