Honestly, when it comes to Exum going at 2 and Vonleh going at #4...I don't think either of those are too big a stretch. Sure it's a significant deviation from what most would predict, but certainly not out of this world.
Regarding Exum at #2
The guys we currently anticipate to go #2 is either Parker or Wiggins. These are two very different prospects:
Parker is the 'safe' pick. He is probably the most NBA ready player in the draft and is almost certain to have a significant impact in the league from day one. On the other hand he is limited defensively, he lacks elite athleticism and his offensive game is already so polished - does he really have that much upside? I think he'll probably be an All-Star at some point, but I doubt he'll ever be a superstar. I can foresee a Joe Johnson type career in his future.
Wiggins is the complete opposite - his draft stock is all about potential. Right now he is a good player, and he'll most likely be able to provide a solid contribution for a team right from the get go...but he needs to bulk up, his offensive game could use improvement (ability to play with both hands, ability to shoot on the move, ball handling, mental aggressiveness, etc). I don't think he'll be dominant in his rookie season, but I think he'll show flashes. With his elite athleticism however, combined with his ability to contribute on both ends of the floor, he has the potential to be an elite player one day. Reminds me a lot of Paul George actually, who is already seen by most as a borderline superstar. But drafting a player on potential means you're taking somebody not based on who they are, but who you hope they might one day become. That means you're taking a risk. Could Wiggins disappoint and become another Jeff Green - merely a 'solid starter'?
Enter Dante Exum.
If the top two teams both decide that an All-Star isn't good enough, and decided to gamble in the hope that they'll pick up a future superstar...they may well both pass on Parker, taking Wiggins and Exum at #1 and #2.
On the other hand, Exum and Wiggins are similar types of prospects. Both are both crazy athletic with good size for their position, both are two-way players who can provide an impact on both ends of the court, both are very good at creating their own offense, both versatile enough to play multiple positions, etc. They have similar weaknesses too - both need to bulk up, both need to improve their jump shots, both are a tad raw, etc. Both guys are projected to go high because of potential, not their current ablities. If a team in the top 2 decides they want to go for potential rather than current ability, but feel they need a guard more than a forward...it's not really that much of a stretch for them to take Exum over Wiggins/Parker.
Regarding Vonleh at #4
Embiid was clearly the top rated big man in this draft class, and it wasn't even close. But, now that Embiid has been red flagged he's a major risk for any team and it's not at all far fetched to see him slip quite far (maybe as low as 6 or even lower, depending on how bad the injury is).
This means that for any team that's after a big man in the draft, Vonleh's stock immediately rises in a big way. Neither Gordon nor Randle is capable of playing center with any effectiveness in the NBA, nor is either one of those guys a capable rim protector. Randle has significant defensive concerns, and Gordon has significant offensive concerns. Neither of them has anything close to a consistent jump shot. Both of those guys are very 'niche' in that they do one or two things really well (Athleticism and Defense for Gordon, Rebunding and Post scoring for Randle) but are otherwise pretty limited. Randle has also been moderately red-flagged for his injury. He insists it's nothing, but that doesn't mean GM's around the league will agree.
Vonleh has the size and mobility (at 6'10", 250lbs, 7'4" wingspan, 36.5" vertical) to play either PF or C effectively. He is a capable Post scorer, he's got a dependable jump shot with range to the three point line, he's a solid free throw shooter (who gets to the line well), he blocked about 2 shots per 40 in college, and he was arguably the best rebounder in all of college last season. He has some question marks (as any prospect does) but he's less flawed than any of the other remaining bigs, and he probably has the highest upside of any big man not named 'Embiid'.
So if one of those top 4 teams decides they really want a big man out of the draft, but doesn't want to take a risk on Embiid, then Vonleh immediately becomes the top choice.