First, I would never let a 4* or 5* hero go. Hero expansion space is cheap. We’ve seen too often a tweak in the hero that makes a bad hero better. I’d feel like a dolt having tossed a now-desireable hero!
Second, the decision is entirely relative to the rest of my bench, and how often you pull epic heroes. Aside from Bane and, perhaps, Brienne, every 4* is better than the 3* tiers if taken to full level. So if you only have a limited set of 4* heroes and don’t draw 4* often, then ascending the best of what you have makes sense.
From my personal perspective, holding all the 4* heroes and many 5*, there is certainly a queue for rare mats. I think some heroes are more valuable for my team than others. And as we have more HOTM and event heroes, some weak heroes fall further back.
But in the spirit of answering the OP’s original question, here’s my bottom of the list, if I had every regular (non-event) hero available:
- Red: 5* Khagan, 4* Kelile
- Yellow: 5* Leonidas, 4* Chao (both good, just everything else is better)
- Purple: 5* Quintus, 4* Tiberius (but tough call here, Grimm and Gormek are better at this role)
- Green: 5* Elkanen, 4* Kashhrek
- Blue: 5* Thorne, 4* Boril
Happy to explain my thinking on any of these. Again, I could imagine using any of these in the right situation if mats and feeders were free, but they’re at the bottom of my wish list.