Puppet 4 has been released and breaks compatibility with version 3. Therefore users will need to upgrade a server to Puppet 4 before updating their clients. Note that if only a Puppet 4 server is available, Puppet 3 clients will not work. Also a number of locations for things have changed including the config, manifest and module files. You will need to follow upgrade instructions for your servers and clients:
Puppet 4.0 enters [community]
May 17, 2015 - Lukas Fleischer