Most of the U.S. military is armed with the M4. There might be some Guard or Reserve units still with M16's but everybody will be armed with M4's eventually.
Agreed, although all are based on Gene Stoner's original design. The M4 was an evolution from the M16 A3/M4. Feed ramps were incorporated into the locking lugs to improve feed performance and the 16" barrel became standard on most models. The M4 is a much improved design over the M16A1 that I carried and much less likely to jam. Some of the late variants have a redesigned extractor as well. Never did quite get over having to turn over my M14 for a M16.
Dave