According to the description of the update from Apple, it contains bug fixes found in previous versions of iOS. The new iOS 13.4.5 fixed a critical issue that allowed tracking users through a VPN connection.
The final version of iOS 13.4.5 should be released within a few weeks. Apple usually tests intermediate minor firmware fairly quickly, so you can expect the release of iOS 13.4.5 in April. New iOS 13.4.5 Download update available for iPhone & iPad Models. The firmware size is about 4 GB. You can download iOS 13.4.5 beta 1 now "over the air". You only need a developer profile.