if the paths are the same, then this might work. But hardcoding the case statements for 4 servers isn't too bad either.
The idea would be to grab the host part of the Host_FileSystem field. Then change your case to match on the path minus the host info. Then depending on which path matches, either /data or /root to the host we grabbed earlier.
But in general, there are probably a few different ways to tackles something like this. And the better you understand the data, the more options you'll likely have.
Again, not tested at all...
index="tablet_os" sourcetype="df" host=dc1* sda3 OR Data
| multikv fields FileSystem, UsePct
| strcat host '@' Filesystem Host_FileSystem
| rex field=Host_FileSystem "(?<path_host>[^/]+)"
| eval Host_FileSystem = case(match(Host_FileSystem,"dev/mapper/localraid-Data"),path_host."/data",match(Host_FileSystem,"dev/sda3"),path_host."/root",1=1,Host_FileSystem)
| timechart span=60m avg(UsePct) by Host_FileSystem
| rename avg(UsePct) as %Use
... View more