Of the 50,000 code groups used only 3800 had been cracked

Nope,by December almost 1000 were cracked.

ope the Japanese imtroduced a new 50,000 group additive
book in August 1941 which set them back months

Question was about April not August.
They also had low priority, Joe Rochefort recorded that his group had
little
equipment, the tabulating machines and teletypes needed had been
diverted to the efforts to break the German and Italian codes and
the Japanese diplomatic cipher

After Pearl Harbor suddenly everything changed and US (and British)code
breakers broke all codes that they were unable to crack in 12 months prior to
Pearl Harbor within a few months .
I wonder how this miracle happened,maybe your famous outer space aliens ,this
time masquerading as US (and British) code breakers made it happen.

He didnt even have a secure comms line to the radio intercept
station. All intercepts had to be delivered by courier. It wasnt
until Febuary 1942 that the group began to read even a small
portion of messages in JN-25

Even according to Safford 7000 codes were enough to read the most JN25B coded
messages and many messages could be read with 1500 codes.
Lets remember US had recovered almost 10000 codes by Dec.4