Somebody more knowledgeable can comment; but my understanding is that the DC
rating much lower because when the switch is operated to break DC current it
is opening a steady state current of 4 amps. With DC also it is important
whether the circuit is inductive/capacitive or purely resistive.
When opening an AC circuit the current (assuming say a sine wave) is rising
then falling to zero twice each AC cycle. Thus ensuring, for example in the
case of say 60 hertz, that the current will be 'quenched' (i.e. at zero)
within a maximum of one 120th of one second. Thus much less chance of
continuing/creating a spark or arcing of the contacts! Also you will rarely
see 'Spark Quench' circuitry on anything but DC contacts/switches, for the
So even though a switch might be able to 'carry' 16 amps through it without
heating up the contacts etc. when it comes to breaking the circuit DC is
very different to AC.
Make any sense?