Q. Is it OK to turn off IPv4 on my Failover Cluster's dedicated networks?

A. Yes. The cluster network (which also hosts Cluster Shared Volume—CSV traffic) and the Live Migration networks are actually tested and written around IPv6 specifically. While IPv4 can be used, the IPv6 protocol is preferred, and disabling IPv4 actually offers advantages. No IP management is required, because IPv6 link local addresses are completely self configuring. The traffic is only for inter-node communication.

Many organizations disable IPv6, but this isn't recommended. Consider that Microsoft does its testing and development focused around IPv6.