Okay here is what you need to do:
1) Run Slitaz 4.0 from a liveCD
2) From the menu, go to "System Tools" -> "Create a LiveUSB" and follow the steps to create a liveUSB
3) from the terminal, cd to your usb device and cd to the "/boot" folder
4) from the terminal, type the following: cat $(ls -r rootfs*.gz) >rootfs.gz
( this will combine rootfs1 to rootfs4 into a single file ... thanks to mojo )
5) copy all of the files from the "/boot" folder of your usb-install into a "/boot" folder in your hard-drive
( you don't acutally need all of the files, but I am trying to make this tutorial easier )
6) Install Grub4dos to the master boot record of your hard-drive and add the following entry:
title Slitaz 4.0 Frugal install
root (hd0,0)
kernel /boot/bzImage rw root=/dev/null home=/dev/sda1
initrd /boot/rootfs.gz
#change (hd0,0) and /dev/sda1 if your boot folder is in a different partition
7) done...reboot your computer
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HOW TO SAVE YOUR SETTINGS
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1) as root user or as superuser (password is "root") , type the following in the terminal:
tazusb writefs gzip
2) use pcmanfm to move the newly created rootfs.gz file from "/" into the "/boot" folder of your hard-drive.
-- while you're at it, you might as well remove the other rootfs*.gz files, since they are not needed and you can remove the "extlinux" folder as well.
....DONE!