Sharp tandem tray MX-DE20

[driving lots of miles]

We have a customer with a sharp MX-3115n that has the tandem tray MX-DE20. The machine will feed out of Tray 3 (tandem left) until empty and then finish the job feeding out of Tray 4 (tandem right), Then the machine will not copy out of tray 4 until tray three is refilled and then copies out of tray 3. The trays are set up identically and the thought is that that tray 4 would keep copying even with tray 3 empty otherwise why have the tandem trays. Am I missing a setting here? Tray 1 is setup for Letterhead, Tray 2 is set for legal, and tray 3 and 4 is set for letter. Any help is appreciated.

[mojorolla]

Check these 2 things:
1. in paper tray settings, are both trays 3 and 4 enabled for print? This is also were auto tray switching is, make sure it is on also.
2. the print driver. If the print job is sent and tray 3 is selected in the paper tab, as opposed to Auto, it will ONLY print from tray 3. Trying setting it to auto and it will match size and type, not source.