I just tried to run the command
kexec memtest86-4.0.iso
To boot into memtest86 using kexec.
This is the output:
Cannot determine the file type of memtest86-4.0.iso
How am I supposed to do this?
kexec doesn't work that way. It boots kernels, not disks.
I think you're trying to boot memtest86, right? Find the standalone memtest86 kernel in the livecd (or from anywhere else) and kexec into that. If you get a gentoo x86 minimal livecd you'll find the memtest86 kernel on it very easily.