From my experience only few environments are Puppet 4, some are still stuck to Puppet 3, most were stuck to Puppet 3 long enough to skip Puppet 4 and directly migrate to Puppet 5.
Except from Timo’s environment I am aware of only one big NETWAYS customer with Puppet 4 and Foreman/Katello.
So I am happy with dropping Puppet 4 if it makes things easier.