Doesn't look like they've added any features with the GTK4 build, just from scrolling through the recent commit history. Much like a lot of these recent GTK4/Libadwaita apps, it looks like they're just maintaining feature parity with the old version while removing the flexibility the older versions had (eg. more packaging options, better customization, none of the reviled "flat button" nonsense, more accessibility options for people with vision impairments, etc).
Kinda annoying, but at least the writing is on the wall that I need to find a new Matrix client.