4 ohm vs 8 ohm

The bottom line is you need to do some math if you want to be certain. The amp is rated to put out a maximum current. The speakers the system came with meet the design criteria of the radio manufacturer. You should be careful when you “load the system” ( connect the speakers) the impedance of you load should match what the system cam with. If you put 4 ohm speakers on an 8 ohm system in a one for one swap and then crank the volume you will burn out your amp. It is no more complicated than that, understand that I’m recently retired form doing this for a living.