Epic Games has released a preview build of Unreal Engine 4.18. This basically sets a bar for shipped features, giving them a bit of time to crush bugs before they recommend developers use it for active projects. This version has quite a few big changes, especially in terms of audio and video media.
WebAssembly is now enabled by default for HTML5.
As for the cool features: Epic is putting a lot of effort in their media framework. This allows for a wider variety of audio and video types (sample rates, sample depths, and so forth) as well as, apparently, more control over timing and playback, including through Blueprints visual scripting (although you could have always made your own Blueprint node anyway). If you’re testing out Unreal Engine 4.18, Epic Games asks that you pay extra attention to this category, reporting any bugs that you find.
Epic has also improved their lighting engine, particularly when using the Skylight lighting object. They also say that Volumetric Lightmaps are also, now, enabled by default. This basically allows dynamic objects to move through a voxel-style grid of lighting values that are baked in the engine, which adds indirect lighting on them without a full run-time GI solution.
The last thing I’ll mention (although there’s a bunch of cool things, including updates to their audio engine and the ability to reference Actors in different levels) is their physics improvements. Their Physics Asset Editor has been reskinned, and the physics engine has been modified. For instance, APEX Destruction has been pulled out of the core engine into a plug-in, and the cloth simulation tools, in the skeletal mesh editor, are no longer experimental.
Unreal Engine 4.18 Preview can be downloaded from the Epic Launcher, but existing projects should be actively developed in 4.17 for a little while longer.