Tricky one !! NFS mount

I have got 4 RH Linux systems with similar configurations . A directory called /ABC is created in all
4 servers . We want to configure in such a way that if we put some files in that dir called /ABC on any one of the machines , that file/s should get reflected in rest of the 3 servers via NFS .
How many minimum NFS services should be running on these servers ? Lets say
we tried to start the NFS services on 2 servers ( 1 & 3 as given below ) . And tried to mount the partitions of 2 and 3 onto 1 as well as partition of 4 onto 3 .
But problem is that with this configuration files are getting reflected only between 1 & 2 and 3 & 4 .

1(NFS) <--------------3 ( NFS)
^ ^
| |
| |
2 4

How do we sort out this problem ? Or any better setup if you have in mind .

Sorry , but the diagram is coming proper ...The arrow which is pointing from server 4 to 1 is actually pointing to Server 3 ( and not 1 ) . Plz bear with this . Thanks

I hope you're not confusing the use of NFS here. NFS lets you mount the filesystem of a server onto the local filesystem of a machine. That would mean that machines 2,3 and 4 would be reading and writing the filesystem of machine 1 in real time.

If you want to just keep copies on each machine, then investigate the use of rsync, which can be run at regular intervals (using cron) to keep all the machines in sync.