A hard disk can only hold 4 primary
partitions (a limitation set when the first PC was conceived in the early eighties). To overcome this it's possible to make one of the partitions extended
. Then that partition is nothing but a container that can hold one or more logical
partitions. Sounds complicated? It gets worse
hda4 is an extended partition (as you can see in the output from fdisk). This means that it holds (in your case) 1 logical partition. You can mount hda5 if you want to. If there's nothing of interest on it, you could delete hda5. Then you could chop up the space in several (logical) partitions. You could also delete hda4 and make a primary partition out of it, but then you can't make any more partitions.
I have probably confused you
totally now. Read this if you are interested in knowing more:
RATS! Two minutes late. We even referred to the same page ...