In the meantime I have tried all AAPL,ig-platform-id's for this CPU.
With and without:
AAPL00,overwrite-no-connect string.
SMBIOS MBP 13,3 / MBP 13,1
framebuffer-fbmem
framebuffer-stolenmem
boot arg igfxonln=1 / -igfxmlr / agdpmod=pikera / gdpmod=vit9696
Is there still a chance to get the 4k monitor running?