Thank you VPhillips for using the forum.
There’s basically two options here: Either your paper quantity sensor on tray 4 is damaged/broken, or you haven't properly specified the paper type settings. For the former, you would need to have service look at the printer, and for the latter, the article found here (http://www.support.xerox.com/go/ke/go.asp?prodID=7800&Xlang=en_US&Xcntry=USA&contentId=x_7800_en-O29...) should help.
You should probably call into support for assistance either way though, as troubleshooting this issue can sometimes get complicated.
We have a Xerox 7800 which is presenting "Paper not availble" from tray 4. Unfortunately, the correct paper size is available. We've removed all paper from the drawer and reset the paper guides. The machine has been rebooted multiple times. All other trays opeate normally.
Any suggestions?