If the 8ohm 3.5" and 4ohm door speaker are both full range, then yes it's correct they are running in parallel and would present a 6ohm nominal load to the amp.
In a 2-way tweeter-woofer system with the speakers run "in parallel" with a capacitor on the tweeter and the woofer wide open, cap on the tweeter will bleed off under a certain frequency, and the woofer will do the same on the upper end due to the inductance of the voice coil. So the amp will not be driving both speakers through the entire frequency spectrum.
In a 2-way tweeter-woofer system with the speakers run "in parallel" with a capacitor on the tweeter and the woofer wide open, cap on the tweeter will bleed off under a certain frequency, and the woofer will do the same on the upper end due to the inductance of the voice coil. So the amp will not be driving both speakers through the entire frequency spectrum.
