Steps to reproduce: Log in to Second Life, go to World > About Land > Environment. There are no altitude numbers by Sky 2, Sky 3, or Sky 4.

We used to have them; they're necessary. While we can look up old screencaps for reference on Mainland (although this should be available in-viewer), private estates can change these and there is now no way to tell at which altitudes Sky 2, Sky 3, and Sky 4 are located.

This is also an issue on Firestorm; not sure if that matters, but it has been reported there previously.