What is 16(4)@250V OF A SWITCH ?

The first is the AC rating, the second is DC.

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 same reason. 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? Terry