I am putting a new stereo in, the new unit and 4 new speakers are 4 ohm, I plan front doors and rear floor speakers to be replaced. Is there any reason if I find 2 good original speakers that I can’t just hook the front and rear doors in parallel? The rear doors are 6 ohm.
Ignore this did some checking and it is not advised.
to speakers in parallel are half the ohms so two 6 ohm speakers would be 3 ohms most good amplifiers would be OK with that but a amp in a radio… id double check with the manufacturer as spec sheets today are either nonexistent or poorly written with missing data
Back in the day when audio was popular full specifications were easily available as was the amps rating into various ohm speakers
No more, today minimal information is given to the buyer
Parnell hookup, ohms stay the same …series hookup, that’s the system that halves the resistance AFAIK.
Perhaps, but I suspect that the Parnell hookup is very rare. 
Two identical speakers (or resistors) in parallel: ohms divided by 2.
Two identical speakers (or resistors) in series: ohms doubled.
Decided just the rear below the seats and front doors only, sounds good.
When a resistive load is connected in parallel, the resistance halves; when connected in series, the resistance doubles. One is better off by combining a series/parallel circuit to keep the resistance at 4 ohms or higher. Another option is to connect a wire wound resistor in series with each floor speaker but this will decrease the volume somewhat.