I was able to play on Realms as recently as 1:00 a.m. EDT this morning. When I tried to log on around 7:30 a.m. EDT, the client updated to 1.9.4, and I got the "Client Incompatible!" Error message when attempting to connect to Realms.
UPDATE: Issue seems to have resolved itself since my reporting of it 8 minutes ago.
Environment
Minecraft 1.9.4 running on MacBook Pro (17-inch, Early 2009) in OS X 10.11.4 El Capitan
Most likely caused by the 1.9.3 client issue.