You did not say whether hda4 is a primary or extended partition.Ok, I think I see what you're saying. I'm pretty sure hda4 is a primary partition, I made it with DiskDruid (the one in Anaconda) out of unallocated disk space. Now, I have no clue how to do what you just said. So, if anyone can help, that'll be great.
AIUI /boot must be in a primary partition for grub to access it.
Also, you can have grub on the MBR for either ubuntu or fedora, but not
both. Then you chainload the other os for booting.
So, AIUI you would have grub for Ubuntu on the MBR and when you
installed Fedora you would install it's grub onto the boot sector for
hda4. Then chainload Fedora from the Ubuntu grub menu. I believe I
read that this will give you a grub menu twice, first from the MBR
version (Ubuntu), then from the boot sector version (Fedora).