ZFS ARC is now claiming 95 percent of available RAM in Backup Server 4.
Both of these use ZFS 2.3, which has changed to take better advantage of unused RAM by default.
See: ZFS 2.3.0 ignores zfs_arc_max, exhausts system memory · Issue #17052 · openzfs/zfs · GitHub
If your PBS 4 isn’t doing that, look in /etc/modprobe.d/ and see if there’s an old zfs.conf file that caps the ARC size. You can comment that out to get the new ZFS 2.3 default behavior.
Note: This is not the new intended behavior for PVE 9, to my understanding. They still want to cap the ARC usage there to make sure VMs actually get RAM when they need it and problems don’t arise from the ARC not releasing RAM fast enough.