I faced this problem too, 12.0.2 mounted nfsv3 (w/o the ",vers=4.2") in read-only mode, and after switching to v 4.2, there was only blackscreen. After debugging the solution for me was to update mount paths, nfsv3 took absolute server path, nfsv4 takes path relative to the exported root:
-ip=::::LE::dhcp boot=NFS=10.0.1.5:/srv/nfs/LE_boot disk=NFS=10.0.1.5:/srv/nfs/LE_disk quiet
+ip=::::LE::dhcp boot=NFS=10.0.1.5:/LE_boot,vers=4.2 disk=NFS=10.0.1.5:/LE_disk,vers=4.2 quiet
Note, that no changes were needed/performed on the NFS server-side.