Let me try to remember, however - do we see a Hellcat VI in WC4, or is it just a Hellcat V?
The roman numeral designates an entirely new class of fighter. In 'real life' the P-47 Tunderbolt (WW2 fighter) has absolutely nothing to do with the A-10 Thunderbolt II (Warthog tank-killer). The Hellcat IV, whatever it may be, probably has absolutely nothing to do with the Hellcat V.
(The exception, of course, is the Dralthi II... but then Halcyon gives the reasons for that designation...)
And yet you're assuming the ALL the fighters we have on Midway's deck are coming out that year? As far as we can tell, the Vampires are the newest fighters coming into production, to judge by the fact that we have to have them brought out to us while we're on a shakedown cruise which turns into a full-fledged mission.
Well, a couple of points...
* The
newest fighter is probably the Wasp interceptor, since it's the "F-110", indicating that it came after the F-108 Panther and the F-109 Vampire.
* Strictly speaking, the reason the Vampires (and Devastators) are brought to us is because the 'elite' squadron assigned them had not yet been assigned to the Midway. They also brought, for instance, Wasps... which we already had several of.
* To be especially anal, the Midway has a Thunderbolt VII squadron -- which certainly isn't new, since they entered service in 2668 (we don't fly them -- but they show up in SO).
I'd suggest that the Piranhas, Tigersharks, and Panthers, among others, are already fighters which are well deployed througout the fleet by this time, ten years after WC4.
Numerically speaking, the F-105 Tigershark and F-106 Piranha were probably entering service around WCIV (in 2673)... since they were assigned designations before the F-107 Lance. Since the Tigershark replaced the Hellcat V, they probably weren't especially widespread as of 2673.
Now, as for the two pairs of craft... it seems more likely that they entered service at the same time -- they were probably part of the same development programme. Afterall, the Shrike/Devestator and the Panther/Vampire are each designated 'Class A' and 'Class B' fighters... and
together the two ships replace a single older ship role.