You need to enable communication between VM1 and VM4 with minimal administrative overhead. Which step should you perform?

Establishing peering between VNET1 and VNET3 is the correct solution. VNet peering connects two Azure Virtual Networks, allowing resources in each VNet to communicate directly using private IP addresses as if they were in the same VNet. This provides a low-latency, high-bandwidth connection with minimal administrative overhead. Creating a user-defined route would be more complex and is generally used for routing traffic through a network virtual appliance or to on-premises networks. Creating an NSG would control traffic flow but not enable communication between separate VNets. Assigning a new IP address to VM4 would not resolve the connectivity issue between different virtual networks.