For circuits resistance is added in series, and divided in parallel. Sometimes it helps to visualize electricity like water through pipes. Voltage is like the water pressure, current is the water flow. If you plumbed 2 lengths of 1/2" pipe in series you would have the pressure drop across the first length of pipe and a lower pressure enter the second pipe and further reduce across it. The amount of water flow would be the same through both pipes.
In parallel circuits, the water flow is divided across both pipes. You would still have the same starting pressure and ending pressure (assume 0). The only difference is that you will flow more water because you have double the pipe cross section.
With the factory speakers rated at 8 ohms you will get x current flow through the radio at a given volume level. If you substitute a 4 ohm speaker the current flow will double, and this increased current flow is what will ruin the factory radio.
Voltage = Water pressure, or the capacity.
Current = Flow - the ability to deliver
Resistance = back pressure or the restriction of the circuit
Resistance in series simply adds. 4 + 2 ohms in series = 6 ohms of resistance.
Resistance in parallel is reciprocal add, example:
a circuit with a 4 ohm and a 2 ohm resistance in parallel adds like this:
1/4 + 1/2 = 3/4. Therefore, the resistance of a 4 ohm and a 2 ohm path is 4/3 ohms, or 1.33 ohms.