Because the 4100 uses the trays to identify paper types and by default should select the highest number tray containing the required paper size first you would need to do one of two things
1) Disable trays 3 & 4 and put the paper size you want as default in Tray 2
2) Put less used paper sizes in 3 & 4 and the default size in 2
This assumes that your trays are reporting their contents correctly - when you move the sliders you will feel them click into place at each of the standard sizes and this should alter how the sensor in the printer interprets their contents.
You can always print a configuration page to see how the trays are being interpreted.
