So the key here is that you’re either going to max the four star, or bring the five star to 3.70. For that reason, I’m going to strongly prefer the four star heroes; they’ll be stronger and you’ll get more usage / better performances from them while you accumulate the materials you’ll need for those four stars.
As others have said, not knowing the rest of your roster makes this difficult, but here goes:
Red: Boldtusk. He’s amazing, his buff is incredibly useful, and while I know you said to focus on offense, he has great utility everywhere (offense, defense, maps, titans). If you felt like there’s too much overlap with Kiril, then Scarlett is a defensible choice.
Yellow: Ranvir if he’s dying / you don’t have Wu Kong. If he’s not alive at critical moments, then increasing his survivability is a strong option.
Green: I don’t know…I guess Margaret since she’s the most offensively-oriented hero? But I don’t love it - do you not have a viable four star option in this element? If not I honestly would maybe wait.
Blue: Kiril is awesome, like Boldtusk. But if you have need of a dispeller (like if you don’t have Caedmon / Melendor), then I think Sonya is reasonable. If you go Sonya I’d definitely go Boldtusk, whereas if you go Kiril here I think that opens the door for Scarlett (though I still prefer Boldtusk).
Purple: I can see a strong option for any of these, depending on your roster. Are you frequently in fights where a lot of status ailments get slapped on you? Then now is the time for Rigard. Though I hope that’s NOT the case since it tilts your roster very heavily towards healing. Do you not have Grimm or Gormek (or Wilbur or Buddy)? Then Tiburtus is a must so you have a defense down hero.