I think whoever is drafting 4th has the toughest decision in the draft to make because of how tight these two players are right now, you could really argue each way.
If i did vote, i would lean towards PLD. And before anyone implies that it is because of his ability to play center, it isn't. I have long stated, that while PLD can play C at a high level, i see him as more effective as a RW and that is where i think he will settle in the NHL (So i see it as Dubois RW vs Tkachuk LW). But the team that drafts him wouldn't be doing their due diligence if they didn't at least try him at C. PLD just has very little weakness in his game but a lot of versatility, in terms of his usage on the ice, can impact the play in all 3 zones, and is continually improving. I don't see the criticism that he has bullied his way through the Q, a lot of his offense is generated off of his smarts, battle, and positioning, rather than brute force.
But on the other hand, Tkachuk is also very smart in the offensive zone, his vision is excellent, and i can see how some might say, offense comes more/very naturally to him. I think he has that "it" factor that is hard to find and that i can't quantify. I have also long argued that he wasn't a leech on the top line and was getting undue hate, he adds a lot to that line and in the few games at the beginning of the season without Marner/Dvorak, he was excellent and was the best player on the Knights IMO, while playing with Berisha/Piccinich or Sherwood. I am glad that his playoff run has opened up some eyes to his ability. But he does have some things to work on like his skating, which will need to get a bit better but i don't think it will hold him game back so much as his game doesn't revolve around speed (i likened his speed/skating style to a Brayden Schenn at this point), he takes a lot of bad penalties (in the playoffs, the joke on twitter was that for every bad penalty he took, he would have two points on the board), strength (needs some more to take the beatings at the NHL level) and a more concerted effort in the defensive zone would be nice (although i saw improvements over the course of the season).
You simply can't go wrong with either at 5. This is why we should keep the pick and take whichever one of these guys is there.