When do 4*s begin to get useful?

Platzy

So, I am one Quicksilver cover away from having all 3*s champed and am now looking towards beginning my glacially paced 4* transition.
(Disclaimer: I am a rather casual player, 607 days into the resupply - My roster)

Now I wonder, which of my 4*s is on the brink of getting useful (or may already be)?
At what coverage do they begin to outshine 3* champions of equal level?
Which ones of mine would work with my existing 3*s at their current cover state?
And what level is the sweet spot for 4*s to level after which Iso costs rise steeply? (I remember for 3*s that was 120)

...or am I still better off collecting a few more covers before considering using the 4*s I have?

Thanks for your advice.

Bowgentle

Looking at your roster, you're either very many covers short or missing a few crucial covers.
Your 4Thor is on the brink of becoming useful, but you need at least one more blue, preferable two.
Others are miles away.

Hoard your ISO for now and try to get 4Thor champed, she's a good one to start your transition.

Don't bother leveling anyone for now, hoard your ISO, then champ right away when you luck into covers.

mckauhu

4*'s are always useful but really useful they began to be around level 135. Most of the characters are then as good as championed 3*'s