[HippoBaro]

As a database engineer who worked extensively with both i3 and i4 instances, I want to add that although i4 has lower IOPS, the latencies distribution of IO ops is an order of magnitude better.

IOPS indeed matters a lot, but so does latency! For our use case, it was much easier to saturate those disks than the old i3s, and we attribute it to the better latencies, making IO scheduling a lot more accurate.