I have found a way to update the g4u image.
I constructed a minimal freedos image to which I added bsdboot.com.
bsdboot.com is able to boot the NetBSD kernel of g4u from dos.
Because g4u consists of multiple floppy images nowdays, I had to make a large floppy disk image. This requires that you must pass the correct geometry of the disk to memdisk.
The correct parameters are given in the memdisk_parameters.txt file, which is available in the archive which you can download from the site mentioned below.
This archive contains a builded image and also the tools to build a new one by yourself.
You need a linux distro to build it.
Read the README.txt from that archive. It explains what all the files do.
for more details.