Are the 4th edition rules available online as a PDF?

They sure are!

Living Rule Reference: https://images-cdn.fantasyflightgames.com/filer_public/3a/fc/3afce41b-b757-4dc8-b005-3a5efffd0fad/ti4_living_rules_reference_v1_1.pdf

Learn to Play: https://images-cdn.fantasyflightgames.com/filer_public/f3/c6/f3c66512-8e19-4f30-a0d4-d7d75701fd37/ti-k0289_learn_to_playcompressed.pdf