A question about sampling with an ADC device
Most likely there is one ADC within the unit and analogue multiplexing is used to sequentially run through all the channels in short time. This means that on the third channel the voltage is sampled and when the multiplexer moves on to the fourth channel, the voltage on the sample and hold capacitor remains unaffected by the open circuit at that input and therefore, the fourth channel appears to be the same as the third channel.

A simple test would be to look at the fourth input with an oscilloscope and see if some portion of the third channel's signal appears during some time period corresponding to when the fourth input channel is "measured".