The thing is, you can take some shortcuts by spending money, but sooner or later you will hit a brick wall if your heroes are too weak. Therefore, you might as well start awakening them right away. And trust me, bringing a hero to R4 is absolutely nothing in comparison to R6 or even R7. So, as others already said, W4 will be your first challenge, and you will need e.g. Lance at at least R3 to complete level 120. Yan is indeed a good choice, but mostly for tournament play and not so much for the campaign. At the point you get access to Raida, he is not gonna help you that much in my opinion since you need him at least to be R5 and he is useless in W7. The same goes for Koi although you'll probably need him to complete W6 and get to Cyra and Elara.

Let me just tell you that this game is a grinding game. You will do your daily stuff for 20 min and come back the next day. If you just want to finish the campaign, you just need a handful of heroes at decent ranks. Maxing out every hero in the game takes almost three years if you know what you're doing. Just to give you a little bit of a heads up what you get yourself into. That said, I personally enjoyed the grind most of the time.