You may want to consider using a utility like OmniDiskSweeper (free) or Daisy Disk ($20) to see if there aren't some unusually large files hiding somewhere, eating up disk space. You can find more apps like these in the answers to How can I free up some disk space?
If your disk is that full and you really can't remove anything else from the startup disk, I would definitely advise upgrading to a larger hard drive. It's generally recommended to keep about 15% of your disk free, and I would certainly advise against getting below 10 GB of free space on your startup disk.
Not having enough disk space on your startup disk can easily cause severe performance problems and more frequent crashing — especially if you completely run out of space.
Other than upgrading your hard drive, you could consider temporarily moving some larger files to another drive until you have enough room for the restore, then, after the restore has completed successfully, you could copy them back to the startup disk.
Depending on what's taking up disk space, it may be worth looking into permanently moving some files to other drives. For example, iTunes and iPhoto libraries, which can often take up a lot of space, can be moved to other drives fairly easily.
When the phone initially said disabled, you could have just left it untouched for a few hours and it will give you the option to try the passcode again, normally has a count down timer to when you can retry the passcode.
Anyhow, you can restore the phone using the DFU mode, hold down the home and the power button until screen goes off then immediately release the power button whilst still holding down the home button, make sure the phone is connected to the computer and have iTunes open when doing this.
The screen will remain black, but you should see the iPhone appear in iTunes where you can restore it.
Best Answer
When connecting it to iTunes, I would choose to set it up as a new device, rather than restoring from a backup.
Once you've done that then do the restore on your iPhone from the backup you wanted to use.
NOTE: Ensure that you do not do a new backup of your iPhone 4s before doing the restore. You may want to make a backup of your iPhone 4s backup before you begin, just in case. If you're not sure of where the backups are located, refer to this article.