My Xerox 4150 keeps taking lots of blank pages through when printing multiple page documents sometimes 30 or 40 at a time

Please correct paper size in the paper tray.No letter ,A4 only.

Two possible solutions to your problem.
If you are talking about grabbing more sheets than one from the tray, you need to make sure the tray guides are up flush against the paper.
If on the other hand you are talking about scanned/copied jobs, check the setting for your scanner, it may be set to letter size which is longer than A4, so the machine thinks it needs another sheet to complete the job causing a blank page to be sent.
Hope this helps