The 4NOT is a simple quad NOT gate / logic inverter, used for inverting clock or gate signals. If the signal at IN1 is 0 (or below 1V) then OUT1 will go high (+5V). If the signal at IN1 is high (above 1V) then OUT1 will output 0V.

Each output is normalised to the next input. Therefore, without a signal plugged into it's IN an output will always be the opposite state of the output above it. So with 5V only at IN1 you will get 0V at OUTs 1 and 3 and 5V at OUTs 2 and 4. In this respect the 4NOT can work as a buffered logic multiple. The 4NOT is essentially just a comparator, so it can turn any analogue voltage into a digital 0/1 signal. For instance you can turn a CV into a stream of gates, or use it to convert any waveform into a pulse wave.

Note, this is not a voltage inverter - it won't make a negative version of a CV or audio signal. For this you need the Takaab 4VIV.