Thx for reporting.
Yes, you´re right:
open the RV_BerlinWS2Planes.dat file and change the line
Axis Germany Fighter BF_109G14AS 400 6000 6000 U3-MK108
into
Axis Germany Fighter BF_109G14AS 400 6000 6000 U4-MK108
that will work. But you can´t load the mission again. You have to start the subcampaign again.
... dgen campaign building is a mercyless business ... every little typo will result in crashing and freezing, blood, sweat and tears ...

the bug ís also in the
RV_G44planes.dat
RV_Berlinwsplanes.dat
RV_Berlinws3planes.dat
RV_G45planes.dat
files ... think i have to update ...