What's the plural of "two", "three" and "four" etc. when they are used as nouns? Should I simply add ans "s" to them or apostrophe + "s"?
Yes. The animals went on board the ark in twos.
"Say, why is everything, either at sixes or at sevens." (G&S-Pinafore)
Except, of course:
two's company, three's a crowd :used to indicate that two people, esp. lovers, should be left alone together.
Yes, four twos make eight, five twos make ten etc.