Relm4 is actually based on gtk-rs, and you will be using gtk-rs widgets all the time.

I have to say however that despite heroic efforts by the authors, it doesn't really feel as nice as the "real" Elm architecture precisely because of the imperative focus of gtk-rs' API, the nature of Rust's memory management system, and depending on what you're doing you may feel forced far too often to drop from fantasy land to reap the benefits of the abstraction.

I'd still consider it next time I tackle a native app, but having tried Slint for writing simple UIs and having read some iced code, I think I'd only go for it if I'm really going for the Gtk native look and ecosystem.

Yep, relm4 is based on gtk4!

What I am asking is, does relm4 have all gtk4 widgets and is generally feature complete? Is there any reason at all to prefer using gtk4 directly rather than the more ergonomic relm4?

You may have to do some bindings yourself, I remember once looking to try out a libadwaita widget and it wasn't fully supported in the available bindings, don't know about the full library of gtk4-specific widgets.

As for feature complete, depends on your definition. It's on 0.6 and major changes do arrive and updates do break user code, but you certainly can write apps with it.

Don't know relm4, but website says its based on GTK as well