Am I supposed to use only 4 stars who are rank 4 from now until fight 16?
There are different ways to do it. Power control, sandbagging, having high ranked champs that can withstand a SP3 or who can take no/reduced damage from a SP3. Once your team is large enough you don't really have to worry about it anymore as you'll have plenty of maxed champs but in the meantime you just need to find what works best for you.
My method might not apply to you. I have 10 maxed 4* and about double of that in 4/40. And about 15 maxed 3*. Streak 1 to 8 I use 4* 1/10. 8 to 15 i bring in my highest pi possible to get thanos/kang team. Streak 16 onwards I play as per normal. Never go below 4* 2/20 or 3* maxed. I can do about 370k per run. So far I'm on 2.59m so far.
Comments
1-10: pretty much any champs.
11-15: 3k PI and above
16-20: pretty much 2.5k PI and above
21+: 1.5k PI and above