After you properly configure Grub4dos entry to boot from an iso image (i.e. in such a way that it allows to boot on modern machines with PAE) for i386 iso of ubuntu 12.04.5 or more recent, you might face the following problem - after loading kernel you get the message:
This kernel requires the following features not present on the CPU:
pae
Unable to boot - please use a kernel appropriate for your CPU.
What modification should be done to the (almost working) grub4dos menu.lst entry:
title Run Ubuntu 16.04.5 x86
find --set-root /ubuntu-16.04.5-desktop-i386.iso
map --read-only --heads=0 --sectors-per-track=0 /ubuntu-16.04.5-desktop-i386.iso (0xff)
map --hook
root (0xff)
kernel /casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper iso-scan/filename=/ubuntu-16.04.5-desktop-i386.iso splash
initrd /casper/initrd