If I understood the logic correctly its works well both ways.
You can have a WMF 4.0 Pull Server supplying configurations to a node that has WMF 5.0 installed.
That’s visible on the LCM of that node, you’ll see it supports v1.0 and v2.0.
Using this will enforce you to use only ConfigurationID as the means for the LCM to know what configuration to ask for.
I haven’t tried a WMF 5.0 pull server supplying configurations for a node that has WMF 4.0 installed
but since the LCM on that node will only support V1.0, id say it should work as long as the configurations you write
and the resources you work with support 4.0. One thing that would definitely not work in this scenario is the ReportingServer
and allowing Push mode nodes to report to it.
Naturally WMF 5.0 on Pull will work best with WMF 5.0 on nodes allowing you the choice between ConfigurationID
That by itself isn’t a reason to upgrade to WMF 5.0, but doing so on the nodes allows MUCH greater PowerShell security
that should be the decisive factor in upgrading it on all nodes.