I have a BT2 installation based on Slax and booting with lilo.conf on partition 1 (hd0,0).
I have free-space of 24Gb at the end of partition 1. You can resize partition 1 if you out of free space.

I boot from BT4 CD and go into X. I start the ubiquity command from a shell console. I select the Manual installation option. I choose my free space to install the BT4 and create the ext3 f/s used by BT4..

The install will also prompt you to install a swap partition.. You can enter the size of the partition on top of the partition list, there is a text box.

The install create /boot/grub/menu.lst for its boot options. It will also create an entry to boot BT2. This is the section in menu.lst
# This entry automatically added by the Debian installer for an existing
# linux installation on /dev/sda1.
title bt (on /dev/sda1)
root (hd0,0)
kernel /boot/vmlinuz root=/dev/sda1 ro vga = 0x317
initrd /boot/splash.initrd

I reboot and select the BT2 installation to boot. It halted with kernel panic-not syncing VFS:unable to mount root fs on unknown-block(0,0)...

I knew my BT2 was still there in partition 1. Somehow Grub is the MBR now overwritten Lilo
created by the BT4 installation..

I have to reinstall Lilo and recover my BT2 partition. I boot from Backtrack 2 USB, with the default lilo boot loader. I chroot into /mnt/sda1 ( my BT2 partition ) ..

From a shell console run
lilo -C /etc/lilo.conf (reusing my existing lilo.conf from /mnt/hda1/etc)

Added *bt...

Lilo gets reinstalled into the MBR. I rebooted and saw the lilo graphical boot screen with bt option. Backtrack 2 reboots fine again.

Now I modify my lilo.conf
cd /etc
vi lilo.conf ( vi ,nano, kWrite whatever from X)

I add the lines..
label = Backtrack 4 ( to the end of lilo.conf )

Next run
lilo -C /etc/lilo.conf ( again )
Added *bt
Added *Backtrack 4

An entry gets added for the Backtrack 4 partittion to boot from lilo which is my preferred boot loader that will also boot the original Backtrack2 partition. (Love that graphical boot screen). But BT4 will not boot from lilo with just that.

I had to reinstall Grub not into the MBR but into the Backtrack 4 partition. The Backtrack 4 installer is not very flexible because it does not offer the option of installing Grub into the same Backtrack 4 partition which could overwrite any existing windows installation with a new MBR.

To do this I boot from BT4 cd again. I check that my backtrack 4 partition is still there and that my backtrack 4 files are still intact. They should be...

Do a fdisk -l
Locate your BT4 partition.
In my case is /dev/sda2 . Note partition table entries is not in disk order but the order they
were installed. For example if you have 2 existing partitions before you installed and created BT4 partition yours might be /dev/sda3..

Next run
Grub > root (hd0, 2 ) which maps to /dev/sda3
Grub > setup (hd0,2 ) to install GRUB stage1,2 into the BT4 partition
Grub > quit you might get some error messages here depending on
the wrong partition number that you incorrectly selected ..

Now reboot lilo. Choose the BackTrack4 option for Boot
You get Grub Loading Stage 1...
Then the original menu.lst options
Choose your BT4 option again and you are there ...
Dual booting BT2 and BT4 with lilo.conf.

This guide cannot have been brought to you without the
guidance of the online grub documentation and to demonstrate that I, stboon can do it ... and express my love for BT4 deviation from the original Slax linux distribution.