yes that is correct, you enable raid on all 4 ports,
then create a RAID 1 Array with SSD1 & 2 as members of that raid.
you can then add more drieves to ports 3&4 without them being members of the raid.
it will work one of 2 ways, either just connect the drives to 3/4 and the will automatically show up in windows as normal. or connect them to 3/4 then add them to a JBOD array / group in the raid setup (just a bunch of drives)