I upgraded a router to 15.4(3)M4, and after rebooting noticed these two files appeared in the flash automatically (at least, I did not notice them before, and their date/time stamps suggest they just recently created):
SWI9X15C_05.05.26.02_00_GENEU-4G_005.010_002-field.spk
SWI9X15C_05.05.26.02_00_AU_005.009_001-field.spk
When I do a "show cellular 0 hardware" the 4G card seems to be running this version of the firmware.
show cellular 0 hardware
Modem Firmware Version = SWI9X15C_05.05.26.02
Modem Firmware built = 2014/08/07 21:13:26