I tend to agree with Tomalak
that the four engines might point to endurance. The writeup in the DS9TM that mentions the four-nacelled version of the Defiant
pathfinder suggests using alternating pairs of warp cores and nacelles. Perhaps the Constellation
was a precursor?
You know, perhaps the Constellation
was backup in the supportive sense. Maybe Starfleet realized it would be a while before enough Excelsior
s were online to supplant the Constitution
class, so Starfleet split out some design directives - let's say, deep space patrol and reconaissance - into another class brief that ultimately became the Constellation