Typically the answer is z2, but I think it really depends on your current training load, your ability recovery and the time/intensity of rides you want to add. If you can add volume and intensity and recover and have solid TR workouts, go for it. If you can’t, then you need to balance additional volume vs intensity. But the main thing is you need to enjoy the mix, whatever it is. If you want some intensity, keep the duration in check so you can recover. If you want to ride for 3-4hrs, keep the intensity down so you can recover. And slowly build up to it.
I do TR LV plans, so roughly 3.5 hours. But I ride 13-15 hours a week. I do the TR workouts during the week and add 2 x 1hr z1 recovery rides (Lazy Mountain and variants or Pettit -1) and 2 3-4hr z2 or free rides on the weekends and might add 1-2 1hr z1/z2 rides during to week to double up on a couple of days. Pushing that volume of z2 on the weekend does not feel easy to me. If I am getting intensity (sustained time above z2) I drop the ride times to make sure I can recover on Monday and be ready to smash a TR VO2 workout on Tuesday. I might hop in a group ride on Saturday or Zwift race / fondo and will not do 4 hours like I would with straight z2. I might also tone down my Sunday ride. I think with a little experimentation and listening to your body you can have your cake and eat it too.
I did Long Fellow on a Saturday during the winter and had done some other 4hr zwift rides that winter. I was a complete mess at the end. 4hrs of z2 with no coasting, no soft pedaling, minimal breaks or stopping is tough.