Ok so I have a jcm 800 4212 combo 50 watt (1986) I want to hook it up to my 4x12 AV cab ,so iv unplugged the jcm 800 speakers and now I'm lost as regards what ohms to I put it into on my 4x 12?4 ,8 or 16 ...there's also an ohm selector on the 800 itself which is 4,8,16... So is it as simple as just matching them up ?

Also when I hook the 2x12 speakers back up after I'm finished using the 4x12 which ohms should I select for the combo which is best ? Cheers !
^ Correct.

If it is a standard 1960AV like this one:
...and the speakers have not been swapped out with something different then it will work in 16 or 4 ohm mono

Set amp accordingly.