Doubts about replication from many servers

Hi,

I have 4 PG servers where each one runs many databases.

Now, I would like to create just one PG backup server to replicate all
the databases from 4 PG servers, is it possible? Or Do I need to create
4 PG backup servers?

And if some PG server goes down, how to recovery the system from PG
backup after I reconfigure the PG original server?