This is a relatively obscure fact about LabPro.

Channel 4 is different from the other three analog channels because CH4 is used for the analog output feature of LabPro. CH4 will read any sensor you plug into it correctly with the following limitation: If you use the +10 to -10 volt Voltage Probe (VP-BTA), the input voltage has to be in the range -5 volts to +5 volts. Voltages outside this range will not be read correctly.