Well the M4 has a new ammunition family, including rounds with reduced back blast for firing in urban, confined spaces. That is a new capability. It can use programable ammunition as well. My understanding is the new ammunition isn't backwards compatible to the M2 and M3.
Part of the issue with Requirements, is too often that a user group focuses on an item not a capability.
If one is properly doing one’s job as Combat Developer a CDD (Combat Development Document) has looked at the needs of the user group/field force, and identifies needs KPP (key performance parameters) of the equipment in terms of both threshold (minimum) and optimal (desired) parameters.
The other goal of a CDD is not to duplicate capabilities, in short don’t make a new requirement if a currently fielded piece of equipment already exists - either in another service, or can be modified by ECP (engineering change proposal) or PIP (Product Improvement Program) to accomplish the task.
With the CarlG the system is already fielded in the M2/M3 configuration with the CAF, so one should be able to conduct an upgrade via PIP to the M4 - writing a new requirement for the M4 isn’t required or desired.
PIP’s and ECP’s can be conducted with O&M money used for maintenance/op stock replacement, while a new program requires one to use the acquisition pipeline for a new program of record.
In short you are looking at years for a new program as opposed to months to get a PIP/ECP conducted.
The other issue with new programs they are supposed to require doctrinal support- something that can be an absolute nightmare that can often lead to chicken/egg discussions that delays things even further.