Help activating available disk space, please.

[Nat Gross]

Hi;
When I think of disk partions, I shiverrrrrrrr. BUT, I need to make
use of disk space I know I have but need to make this safely. Please
adivse.
Following, is the result of fdisk -l:
=======================
Disk /dev/hda: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks        Id       System
/dev/hda1   *           1          13         104391        83       Linux
/dev/hda2              14        3837    30716280     83    Linux
/dev/hda3            9473        9726     2040255     82   Linux swap / Solaris
/dev/hda4            3838        9472    45263137+   5  Extended
/dev/hda5            3838        8102    34258581    83   Linux

Partition table entries are not in disk order
==========================================
[First] Does this mean that I have but one drive? (thought I had two.)
Now, based on the following output from df, I *think* that hda4 is
available for use.
If so, how do I verify thats its empty, if it has a filesystem, and
why is its block count larger than hda5 which brings the total gigs to
far more than the 80G the drive has.

df -h
=================================
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda2              29G   26G  2.3G  92% /
none                  506M     0  506M   0% /dev/shm
/dev/hda5              33G   21G   11G  66% /usr
=============================================

df
===========================================
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda2             30233928  26342300   2355816  92% /
none                    517176         0    517176   0% /dev/shm
/dev/hda5             33720764  21005124  11002712  66% /usr
===============================================

Thank you, all.
-nat