FastLED 4.0 will consist of multiple parts:

Fx engine. This library is designed to make fastled more like WLED and give app features and close the gap a bit. Fx pieces can fit like plugins and code can be shared and integrated back into the engine without requiring that user copy the entire sketch and slice out the parts they need. Over and over again in the examples I see lots of boilerplate and the graphics algorithm.

Fastled wasm compiler. I think we all hate deploying code to the physical device and watching the serial monitor. It's slow, it's buggy. I do certain things which puts the device into a weird state. I can't even upload code to the ESP32-C3 on Windows 10. The FastLED compiler is designed to side step all of that and deploy it in the browser during rapid development. Then as a final step you deploy it to the actual device and things work as expected. If you haven't seen it yet, then I suggest you try it out. Here's how you can install it: pip install fastled. Then go to your sketch and run fastled at the root. Compile times are on par with wowki at the moment. However, I can make it much faster. Right now it seems "fast enough". This one really is the most important out of everything for 4.0. You can visit the repo here: https://github.com/zackees/fastled-wasm. You'll notice that there are a lot more UIButtons like sliders and stuff. This is mostly to feed into the browser version of FastLED. The first sensor I added, which is the PIR, contains a UIButton so that users can trigger the sensor in the browser manually. This is great and use it all the time for my personal art work.

Video. I love video and i think it's the future. I've been making a lot of projects with it. With 4.0 there will be a release of my other project ledmapper.com, which I will integrate into the fastled page. It produces mapped video on led matrices and also irregular surfaces designed specifically for FastLED. It blew people's minds 10 years ago when I was fooling around with prototypes. I do think that Video will be the killer feature that finally gives artists a reason to use FastLED instead of WLED.