Although I don't have an R4B (2 R-4C's, though) I'm looking at several
front panel pictures of the R4B. It appears to me that, just like on
the T4XC, there's a small dot on the screened front panel where the mic
jack would have been punched as on previous models. On the T4XC model,
they moved the mic jack to the rear panel, owing to the crowded
conditions behind the front panel. One might assume, then, that a
similar condition existed on the R4B but a previous owner may have none
the less simply moved/installed (in this case) a phone jack on the panel
for convenience. The same thing can be done to the T4XC, but its a
tight fit, inside!

Rob Mills wrote:

I just acquired an R-4b (serial # 9737C) and it has a 1/4 phone jack between
the RF gain knob and AVC switch. Don't see this jack in any info that I can
locate.The receiver has been stored un-used since being re-caped and etc by
drake just before drake quit taking in and rebuilding things, probably 8 or
9 years ago. Would drake have added something? The relative who owned it was
not inclined to modify equipment. RM~

PS, No I haven't opened it up or fired it up yet, just un-boxed and studying
things.