Due to limitations in the APT repository software that we use (reprepro), our APT repositories can only contain a single version of a package. When we published Passenger 5.0.2, Passenger 4 was no longer in our APT repositories.
However, there are users who still wish to have access to Passenger 4. Because Passenger 5 introduces a few breaking changes (e.g. renames of some configuration options, dropped Rails < 2.3 support), some users need more time to upgrade.
We've published a repository specifically for Passenger 4. You can use this repository by modifying the APT URL in /etc/apt/sources.list.d/passenger.list.
Passenger open source:
deb https://oss-binaries.phusionpassenger.com/apt/passenger/4 <CODENAME> main
deb https://download:YOUR_DOWNLOAD_TOKEN@www.phusionpassenger.com/enterprise_apt/4 <CODENAME> main
<CODENAME> with your distribution's name, e.g. "trusty", "wheezy", etc.