This is an expected result, and it is not specific to Ceph. Any storage that consists of multiple disks will produce a performance gain over a single disk only if the workload allows for concurrent use of these disks - which is not the case with your 4K benchmark due to the de-facto missing readahead.
I don't think that affects writes.
To allow for faster linear reads and writes, please create a file, /etc/udev/rules.d/80-rbd.rules, with the following contents (assuming that the VM sees the RBD as /dev/sda):
I think the OP didn't say that the client is a VM. It might be, it might be a KRBD mount, it might be through CSI, we don't know. I asked the OP privately if it's a VM because the first thing I check in this situation is libvirt / librbd throttling. It sounds a *lot* like saturating an IOPS client throttle, something I've seen time and again in virtualization scenarios.