I just installed FC4 as a 3rd OS with Suse and XP.
Suse wrote the grub.conf file so I didn't want to mess that up because it works perfect. So here it is:
Put in FC4 disk 1 or DVD; when the Anaconda installer gets to partitioning, left click on the 20 gigs unallocated; click "New"; select ext3 for filesystem and / for mount point. You could also create a separate partition for /home if you want (I did not). Sounds like you have a SWAP already with Ubuntu. FC4 can share it with Ubuntu (do nothing with SWAP). Next, when you get to GRUB, click on advanced; then choose "No Bootloader". Then you should be directed to pkg selection and final install.
Once install is complete, boot into Unbuntu, get to /boot/grub/grub.conf or menu.lst (whichever Ubuntu uses), edit the file and add the FC4 boot entry:
title Fedora Core (2.6.11-1.1369_FC4)
kernel /boot/vmlinuz-2.6.11-1.1369_FC4 ro root=LABEL=/ rhgb quiet
Remember that you must change the root (hd1,4) for your particular partitioning scheme. In my case, FC4 root (/) is on /dev/hdb5 which is (hd1,4) in grub language.
After you make that entry reboot and choose FC4 from the GRUB menu and you should be in.