Hi Gianluca,
As mentioned in below RFE, EL 8.4 is not enough, you also need Advanced Virtualization 8.4 (libvirt >= 7.0.0) and AFAIK this has not been released upstream:
So right now you can use 4.6 cluster level only on hosts running on CentOS Stream.
Also the error message tells you that all your Up hosts need to provide EL + AV 8.4 features before you upgrade cluster level. But of course you can upgrade them one by one, it's not required to put all hosts into Maintenance before upgrade.
Regards,
Martin