Yum install Puppet 4.x on a CentOS 6 machine

You replaced the wrong EPEL repo (for CentOS 7) with the correct repo (for CentOS 6) but the system still is trying to download EPEL packages for 7.

In this case yum still has cached metadata from the EPEL repo from the last run with the wrong repo.

To resolve the problem, clear the cached metadata.

yum clean metadata

From the Debian backports documentation, you need to manually specify the repository when you install via the CLI, because backported packages are pinned with a higher priority than default packages.

As per this ServerFault answer, you will need to pin either the entire repository or individual packages to use backports.

With the Puppetlabs apt module, you can do this do this using the apt::backports class, which will both add the backports repository and drop a pin file:

class { 'apt::backports':
  location => 'http://ftp.de.debian.org/debian',
  release  => 'wheezy-backports',
  repos    => 'main',
  pin      => 500,
}

If you don't want to pin the entire repository, you can pin individual packages using apt::pin (the below is an example, you may need to tweak it):

apt::pin { 'backports_git':
  packages => 'git',
  priority => 500,
  release  => 'main',
}

More documentation is here: https://forge.puppetlabs.com/puppetlabs/apt#pin-a-specific-release