Apple today released the iOS 12.4 beta 7 update. It's available to both registered developers and public beta testers, so if you're in either program, you should be able to grab the over the air update now.
There haven't been any major iOS 12.4 features announced or discovered quite yet. With the Apple Card slated to launch this summer, support for that credit card could be baked in to the update, but it's looking like iOS 12.4 will largely be focused on bug fixes or performance improvements. It'll likely be the last major iOS 12 update before Apple moves on to iOS 13, so it makes sense that Apple would be focusing on ironing out the last few iOS 12 kinks.
Also released today is watchOS 5.3 beta 6. Once again, there haven't been any major features found in the beta releases so far. It's likely that this update is also focused on fixing bugs and smoothing out performance before Apple launches watchOS 6 to the public this fall.