I was actually about to post a similar question. My 4* transition has badly stalled with vaulting. I have three champed 4*'s (Devil Dinosaur, Nick Fury and Star Lord), but otherwise I have almost a dozen characters in the 10-12 cover range, but only two are currently in tokens (Wasp w/12 covers 5/3/4, but being vaulted in the next 24 hrs and Mordo w/10 covers 2/4/4). All but two (Mordo and Thor) of my 10+ cover 4*'s have at least one ability at 5 covers, making BH a bit of a risk for wasted covers.
In the next couple weeks, it would appear that 4* Thor's red will be in the Hero for Hire package, which would put my 3/4/4 Thor only one cover or any color from being champed (all three of my current 12-cover 4*'s, Ant-Man, Wasp & X-23, have 5 covers in their first ability, so HfH doesn't allow me to champ right away unless it rotates later on). With my current 4* champs, I've able to hit progression in PVE with my scaling, but never place in the Top 100 due to inability/unwillingness to spend the time. In PVP, however, my MMR makes getting even remotely close to 575 extremely difficult unless either Devil Dino or Star Lord are boosted, as I'm matched up against teams of better, boosted 4* champs and 5*'s up to ~Lv420, even when none of my 4*'s are boosted.
I feel like a champed 4Thor/~Lv200 IM40 team could be quick/effective enough for climbs to 575 for the CP, even punching above my weight. Unfortunately, I'm in the 3,600 HfH group, so that would basically drain 90% of my available HP (after accounting for roster slots for Gamora's 4* and Rocket & Groot's 4*).
What do people think? Would 4Thor be worth the 3,600 HP investment or would she make less of a difference than I expect? If it matters, 4Thor is currently my only BH and I've been averaging about 1 4* BH a month.