[wplug] Bash scripting help - count from 1 to 4

[Gentgeen]

I am writing a bash script as part of my backup routine. What I need 
to keep track of is which zip disk to use, Disk 1, 2, 3, or 4. 
(rotating the disks out once a week)

Now I want to have a file, called DISK that will just contain one of
those numbers. What I want the script to do is something like this:

   1) If file DISK does not exist, exit with 
        echo "Don't know the disk # in the drive"
   2) If DISK # is 1,2,or 3 then increase it by 1  
        (so if the file contained a "1" now it will contain a "2")
   3) If DISK # is 4 then reset by going back to 1
   4) Give a variable to be used used later in the script with the
        new number.
        echo "Please replace old disk with Zip Disk number $NUMBER"
       

Any help would be appreciated.
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