Depending on your needs, there may not be a compelling argument to switch to 4 Gbit. People, even the vendors, get too focused on the bandwidth characteristics of Fibre Channel (FC). There's an obsession with bandwidth in general.
Going to a faster bandwidth may yield benefits in reduced latency, allowing time-sensitive applications to run faster. Similarly, the need to aggregate might allow a switch to fan in several slower servers to a faster 4 Gbit pipe to a 4 Gbit storage device.