The 4100 will look at tray 1 first then tray 2 If tray 1 is empty it will pull from tray 2 if the papersize matches the job otherwise back to tray 1 and ask for the paper.
Your machine likely thinks the paper in tray 2 is not the size you want. For instance you have LETTER in tray2 But the machine thinks it is legal.
The LJ 4100 detects the paper size in tray 2 automatically. If you print
a configuration page:
MENU them INFORMATION then ITEM then CONFIGURATION PAGE then SELECT.
The page will print and show what size paper the machine thinks is in Tray 2 if it says LEGAL and you have LETTER you need to Check The Paper Backstop in TRAY 2.
The back stop sets the length of the paper if it is not at Letter (8 1/2 x 11) the machine will not detect LETTER. Even if it just a little bit off or between positions.
If it is set at LETTER then you may have bad switches in the machine that are operated by the tray.
Or the arms on the tray that operate the switches are not moving properly.
You will find the arms on the rt, side of the tray and feel them change position when you move the backstop.
Hope this helps.