That /U4 is used in different places for different things. "/U" is for "Umbau" (only in factory)
The most important from my point of view is that in the danish website
www.ww2.dk and the OoB's there.
In these tables, I am 100% sure that the
/U4 means the central MK108.
I.e. II/JG52 gets these "109G-6/U4" in November 1944, which fits very good to the date given in Lipfert's diary. Also, Lipfert mentions one JG77 Gruppe which had this gun before. This can also be verified on the website.
The
MG151/20 gondolas are definitely called "/R6" for Rüstsatz (changeable in field)