So, ``4-terminal network'' is a generic thing representing a bunch of circuit elements in some network (it could be pretty much any configuration) with two ``input'' terminals and two ``output'' terminals. Often one plugs in a power supply to the input, and the output is the ``business end'', where one is interested in the voltage. However you could arrange more than one network together, or reverse input and output. We looked at one specific example today and we'll see lots more examples. We'll usually be considering these in an AC context, i.e., assuming some sinusoidal steady-state driving, rather than with transients.