Zero lvl 1 says that you can place a jack Skelington in your discard pile into play, without using his comes into play effect, and that the action DOES NOT count as a play. Does this mean you could play a Jack lvl 4 even though you have no lvl 3 friends in play?

See, it doesn't count as a play according to Zero's text, so I don't believe the normal friend level up play restrictions apply.

Is this incorrect?

Yes, you could do that.

This effect would also work for Dr. Finkelstein's power, which has both players put up to 3 friends into play. They can also be of any level regardless of your friend build.