I have made some tests with a new 4MLinux ISO image. It works as follows:
1) When I have BIOS enabled, the CD uses the 32-bit kernel to boot.
2) When I have UEFI enabled, the CD uses the 64-bit kernel to boot.
Both the kernels boot exactly the same 32-bit 4MLinux (!).

As far as I know, this is the only hybrid ISO, which works this way. The results are very promising, so I plan to add full 64-bit/UEFI support in 4MLinux 20.0 (both for running 4MLinux as a live CD and for installing it to a hard disk drive). Once again: this will still be a 32-bit system.

4MLinux 20.0 BETA should be ready in September 2016.

.

UPDATE:
I am slowly finishing my work on 4MLinux 20. It has just one 32-bit kernel.

LiveCD can be booted on the following machines:
BIOS with 32-bit CPU,
BIOS with 64-bit CPU,
UEFI with 64-bit firmware.

After installation to a hard disk drive, the following are supported:
BIOS with 32-bit CPU,
BIOS with 64-bit CPU,
UEFI with 32-bit firmware,
UEFI with 64-bit firmware.

BIOS compatibility in UEFI is only required to install 4MLinux to a HDD.
When the installation is over, this compatibility can be switched off.

I will update the 4MLinux documentation in mid-September, when 20.0 BETA is officially released.

.