It all depends what mats you have to ascend your 5* heroes. A fully maxed 4* is better than a 5* stuck at level two or three. I would start by looking to focus on 4* teams. One for defence, one for offence, and a Bench for Titan and trials etc.
Start with your best 4* heroes which in my opinion are - Wu Kong, Guardian Jackal, Boldtusk, Hansel, Rigard, Kiril, Grimm, Li Xiu, Caedmon, Gretel, Colen, Scarlett, Kashhrek, Gormek, Sabina, Boril, G.Falcon.
I agree with focusing on your 4* for now. Remember you need to build deep benches for alliance wars (if you are in one). Also 4* are much quicker to max and very effective. In fact they are the backbone of the game. Once you have a selection of 4s they will help you heaps with collecting materials for your hungry 5s.
The most powerful 4s are Wu Kong, Proteus, and Wilbur (my opinion). Proteus will get you through many rare quests and challenge event levels (for ascension mats) that you would otherwise have no chance of completing until you finish leveling a full 4+ team. It’s not necessary to always go rainbow, you already have Aeron and Frida well along the way to 2/60. Having a secondary heal is very helpful as well as the element link that benefits Proteus. I would finish those two along with Boldtusk, Wu, and Proteus.