IMHO the two best gap fillers between the G-6 (no MW50) and K-4 would be :
G-14 : basically G-6 with MW 50, produced from mid-44 onwards with a max speed of 665 kph at 5000m, and 568kph at SL. iow, it`s a good `44ish medium altitude fighter.
G-10 : Since we already have it, and as it could represent the other important 44/45 109, the G-14/AS. The performance diffo between the G-14/AS and G-10 is marginal, ie.
early G-10 : 562 at SL, 690 at 7.5km (1.75ata)
G-14/AS : 560 kph at SL, 680 at 7.5kph (1.7ata)
It speaks for the G-10 that it`s
a, already ready
b, in greater numbers, some 2600 being produced
So`d have for 44/45
G-6 : 530 SL / 640 at 6.6 km
G-14 : 568SL / 665 at 5km, above as G-6
G-10 : 562 SL / 690 at 7.5 km, much improved altitude performance over G-6
K-4 : either :
1.8ata : 595 kph SL / 712 kpt at 7.5 kph
1.98ata: 607 kph SL, / 715 kph at 6km, above that as 1.8ata,
Has MK 108 as standard and only gun