You're mistaken
The M-16A4 is the current model being used by the majority of the armed services. For the most part, only special forces and some regular units (depending on the need and availability) are using the M-4. The Marine Corps will be the primary user of the M-4 in the future as the Army are looking to transition to whatever ends up coming out of the OICW/Individual Carbine (and related) projects. However, most of the rifles in service today are still M-16s - generally A4s. The M-4 isn't all that new itself.. it has been around for close to 20 years now.