Apple today rolled out iOS 12.1.3 beta 4 to both registered developers and public testers in the Apple Beta Software Program. If you fit into either group, you can grab this update by going into Settings > General > Software Update on your iPhone or iPad.
There haven't been any major new features or anything like that discovered in iOS 12.1.3 so far, so this update may be focused on fixing bugs and improving performance under the hood.
Apple hasn't said when iOS 12.1.4 will be released to the public. Now that we're several beta releases in, though, a public release could happen at any time. Stay tuned and we'll let you know when the update is made available to everyone.