Enable persistent grants on c4.8xlarge

D2 instances provide the best disk performance when you use a Linux kernel that supports persistent grants, an extension to the Xen block ring protocol that significantly improves disk throughput and scalability. For more information about persistent grants, see this article in the Xen Project Blog.

EBS-optimized instances enable you to get consistently high performance for your EBS volumes by eliminating contention between Amazon EBS I/O and other network traffic from your instance. D2 and H1 instances are EBS-optimized by default at no additional cost. For more information, see Amazon EBS–Optimized Instances.

C4.8xlarge is already EBS-optimized by default, when you create the instance, during the third step "Step 3: Configure Instance Details", you should be able to see checkbox "Launch as EBS-optimized instance" as been selected and you cannot deselect it.

For the current list of EBS Optimized instances type, here is the page, just for example, as g2.2xlarge is not EBS optimised, you can select the checkbox "Launch as EBS-optimized instance", and additional charges will be applied.

If you have a more specific requirement or details from your customer, please share more information.