You can use fdisk (fdisk /dev/(device name without any numbers)
then m brings up the menu and that may help from there
another option, if you have it, is parted or gparted. That may help at this point, as it is graphical and could assist you with how you would like to partition
One thing to consider, is that you are only allowed 4 primary partitions. You broke convention with your implementation, using /dev/hda4 as a data partition
You can use an extended partition, which technically, uses a primary partition to create a container for logical partitions.
You would have /dev/hda1, hda2, hda3 (hda4 would be the remaining disk, but unused) then hda5 and onward would be logical partitions that are contained in hda4.
This may help
Large Disk HOWTO: Extended and logical partitions