It depends how far you rank up. As they are your only 4 stars, you should upgrade all of them till 3/30. Personally i preffer not to upgrade any of them after that. Wait till you get better.
In tech class ultron, vision, starlord
In mystic class guillotine, magic, ghostrider, voodoo
In skill class gwenpool, blackpanther, electra, hawkeye
At your level based off the post if that's all you got then rank them up as far as you can with the materials you have(other than t4cc). Eventually you become very selective on who goes up to 4/40 and some won't even qualify for 3/30 but that comes when you have a strong stockpile of 4/5 star champs.
But if you only have 3 4 stars than just ranking people up so you have champs strong enough to beat content and get more materials and champs should be your focus
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In tech class ultron, vision, starlord
In mystic class guillotine, magic, ghostrider, voodoo
In skill class gwenpool, blackpanther, electra, hawkeye
But if you only have 3 4 stars than just ranking people up so you have champs strong enough to beat content and get more materials and champs should be your focus