NBA Draft Hypothetical: Who would you pick at #4 if the Celtics traded up with the Knicks?

I don't have any rumor to report today so I thought I'd do a thought exercise.  There are some rumblings that the Knicks would be willing (or eager?) to trade the #4 pick in order to gather additional assets.

Knicks likely to trade No. 4 pick in 2015 Draft if Okafor unavailable

If Okafor is gone, somebody is going to take the No. 4 pick from the Knicks. This is the intel I am hearing from points near and far as we count down the days to the 2015 NBA Draft. Phil Jackson has a bare cupboard when it comes to future draft picks, and he is inclined to trade down in exchange for multiple picks in this year’s draft and in future drafts.

So, hypothetically if the Celtics decided to throw enough picks at the Knicks to move up to the number 4 spot in the draft, I think that gives the Celtics an interesting (but good) dilemma. (Note: the question about how much it would cost to move up to 4 and how much it would be worth to do so is a related but different set of questions).

It is generally believed that Karl-Anthony Towns, Jahlil Okafor, and D'Angelo Russell will go in the top 3 picks (though there's been a recent push by Kristaps Porzingis and Emmanuel Mudiay could be an option for some teams).

Chad Ford's annual survey of GMs seems to indicate that the next tier of players includes Willie Cauley-Stein, Mario Hezonja, and Justise Winslow.

Assuming the top 3 I mentioned do go 1-2-3 in the draft, who should the Celtics pick at number 4?