Steve,
I agree this is a problem area for upgrading to 4.0 and it's the one place where some manual checking or updates may be necessary.
Unfortunately it's hard to automatically split up $Conf{RsyncClientCmd} into $Conf{RsyncSshArgs} and $Conf{RsyncClientPath}.
Also, $Conf{RsyncClientPath} is now somewhat misnamed - it can be any command including sudo, nice or caffeinate. It should be the rest of the old $Conf{RsyncClientCmd} after the original ssh arguments:
$Conf{RsyncClientPath} = '/usr/bin/sudo /usr/bin/caffeinate -ism /usr/local/bin/rsync';
Craig