iOS 12.4 beta 6 is now available. If you've registered with Apple as a developer, you can grab the over the air update by going into Settings > General > Software Update on your iPhone, iPad, or iPod touch. It's also available through Apple's Developer Center on the web.
Public testers in the Apple Beta Software Program should get the update within the next day or two.
The update to iOS 12.4 isn't expected to include any major new features. The one notable change that'll probably be included is support for the Apple Card credit card, which is slated to launch this summer.
There's no word yet on when iOS 12.4 will launch to the public. Apple typically has around six beta releases of an iOS update before its official launch, so we could see an iOS 12.4 release any day now.
Apple today also released watchOS 5.3 beta 5 for developers. There haven't been any major new features found in this update, so it may be focused on bug fixes and under-the-hood performance improvements. Apple hasn't announced when it will release watchOS 5.3 to the public.