I used XT30 as my primary camera since it has been released. I was renting XT3 occasionally when I needed 2 camera setup, but for me XT30 was absolutely and totally enough covering all my needs. I don't film any documentary or weddings so XT30 record limits are totally ok for me. It if perfect in filming 120p b-rolls, prime optics suits it well, I was totally satisfied...
...Until I bought XT4. IBIS is something amazing in shooting handheld b-rolls (I have Ronin SC as well, but I don't use it in closeup b-rolls). My XT30 b-rolls were smooth but XT4 made them REALLY SMOOTH.
Comparing to XT30, XT4 has a strange screen flip mechanism (I'd rather have the one they put on XT3). This mechanism is only ok when shooting vertical instagram videos.
Now I have 2 camera setup which is perfect for me. Yes, XT4 is worth upgrade for video from both XT30 or XT3.