Be sure which device you are actually trying to mount. From what you have stated, the hdc4 device should be the Master device on the Second IDE channel/cable. You are trying to mount the 4th PRIMARY partition (not a logical partition, those start at 5). The reasons I mention this is that most people do not have 4 primary partitions on a hard drive unless they explicitly did so.
Just a quick recap on IDE channels and partitions:
-There are 2 IDE channel/cable locations, these plug directly into your motherboard.
-There are 2 device locations on each ide channel, a master and a slave.
-So you have a total of 4 potential devices, hda, hdb, hdc, hdd
hda-master device on first ide-cable
hdb-slave device on first ide-cable
hdc-master device on second ide-cable
hdd-slave device on second ide-cable
Usually these are a mix of cdroms, hard drives, etc. If you have an ATA disc controller (usually a card that plugs into a pci slot that gives you extra ide slots) that will add additional devices starting with hde. The physical location of the device is what matters, not necessarily how many you have. I.e. if you have a cdrom and a hard drive each on a separate cable as master devices, you will have an hda and hdc device. If they were on the same cable that plugs into the first ide slot, then you'd have an hda and hdb device.
on a given hard disk you have 4 possible primary partitions and any number of logical partitions (i'm sure there is an upper limit, but I don't know what it is).
Even if you only have 2 partitions, the type of partition affects what they are called. Say if you made 1 primary and 2 logical partitions on a single hard drive in the master position on the first IDE cable, they would be referred to as:
hda1 - primary
hda5 - first logical
hda6 - second logical
If it was on the second IDE cable as the master device (second connector on motherboard, first connector on the cable), you would refer to them as:
hdc1 - primary
hdc5 - first logical
hdc6 - second logical
So, to recap from what you said. hdc4 is the fourth primary parition on your hard drive, and your hard drive is in the physical location of the master device on the second IDE channel/cable.
I believe you are most likely trying to access the wrong partition or the wrong device.