Just for the case that anyone is reading this, and has problems getting the binary of Memtest86+ onto a floppy like me, there are some things to try.

- If you run the script "install" or "install64" from the windows explorer, you will be unlikely to succeed as the script will not be able to find the files that are located in the exact same directory. I assume this has something to do with the over-observing Windows Defender of Win10/Win11. To solve this, launch a command prompt with administrative rights, navigate to the "floppy" folder of the desired Memtest version that has been unzipped before, and launch the command from there.

- I could not get any of the Memtest86+ images onto a floppy, it always said "Error 86 Wrong Parameter". I managed to find out that usually if writing a floppy image with "dd.exe" the "block size" should be specified with 1440k for 1.44MBytes on a standard 3.5 inch 2HD floppy. I don't know why but for all the install.bat and install64.bat scripts, the creator has specified a block size of 18k. If you right click on the batch file, and edit the command

dd.exe if=memtestp.bin of=\\.\a: bs=18k

to

dd.exe if=memtestp.bin of=\\.\a: bs=1440k

it worked for me. (There was still an error message 87 Wrong Parameter, however the created floppy was able to boot up and run Memtest with no problems on my DX4).

the memtest86 (non-plus) 4.1 iso version has multiboot setup, the 3.5b version works on 486dx4 (i did not test the 3.5b directly).
another interesting thing, i could write out on windows xp to an sdcard with rufus. then it could boot on a 486 with an sd-ide adapter, no floppy needed.