I am pretty sure we could live comfortably on a db.r4.4xlarge, but we have some bursty analytics applications that sometimes bring our load up for about a minute, and we like the peace of mind of headroom during peak hours (we are in edtech with super predictable traffic throughout US school hours).
Total side note: I just went to the pricing page and it looks like they just released db.r5 instances for Aurora (https://aws.amazon.com/rds/aurora/pricing/)! It might be time to try those out. I don't see an announcement just yet though...