Some items of interest from the release notes:
Backup and Restore
Fixed in beta 5
A restoration of an iCloud backup onto the same device the backup was taken from may not work properly. As a result, some apps may crash.
Known Issues
Encrypted backups to iTunes fail.
iOS 7 iCloud backups restored to an iOS 8 beta device may not properly restore photos.
Handoff
Note
Handoff is incompatible between devices using this beta and the prior beta. You should test Handoff with the most recent beta software because it is not backward-compatible with previous betas.
iTunes Sync
Known Issue
Apps do not sync from a device to iTunes.
Music
Fixed in beta 5
The Music app may stop responding when downloading an album.
Settings
Known Issue
Some icons are missing in Settings and only appear after the row is tapped.
Setup
Known Issue
Updating the iCloud settings screen may appear to hang during setup.
Workaround: Some accounts may have an extra long lag. Wait a couple of minutes and they should complete.