Please note that in this issue the term "workspace" in p4 overlaps with the same term in jenkins. I'll try to be clear which I mean.
While the p4 plugin lets the user manually configure the p4 workspace, it does not let the user specify the p4 workspace root. The plugin automatically uses the jenkins workspace directory as the p4 root.
This is a problem because it means that the user cannot "automatic cleanup and sync" or "forced cleanup and sync" without destroying every file in the jenkins workspace. This is not desirable if we are using multiple SCMs, or other configurations where we only want the p4 plugin to clean up the p4 tree.