Trading 4 for one is very rarely a good idea.
Based on what?
Because nobody in a draft class has played a single NBA game, so every single player carries some element of risk.
Pretty much every single draft you;'re going to have at least one or two guys in the top 10 who end up being absolute garbage, and you end up having at least one or two guys between 15-30 who end up being solid starters or at the very least good role players.
If you ONLY draft one guy in the top 10, then you are putting all of your hope in one person - if that guy busts, then your entire draft is a bust.
If you draft four guys, then every one of those guys offers at least SOME chance of becoming a solid rotation player (or better) in the NBA...so your chances of getting a useful player increase significantly.
For all we know right now, Okafor could turn out to be a taller Anthony Bennett. Rozier could turn out to be the next Eric Bledsoe. Hunter could turn out to be the next Richard Hamilton. Mickey could turn out to be the next Elton Brand. Thornton could become the next Leandro Barbosa. Every one of those comparison players were great in their primes, and if you get a player as good as any of those out of a draft (even from the top 10) then you're laughing.
Now the chance of hitting on all four of those? Highly unlikely. The chance of hitting on one, or even two, of those? Pretty decent.
That's why unless you have a prospect that has franchise player potential (a Tim Duncan, Lebron James, Anthony Davis type) then you're often better taking multiple hits rather than one hit.
It's like buying a box of high end basketball cards at $500, versus a case of mid-range basketball cards for $500. That high end box for $500 is going to be hit or miss - either you get a super valuable card, or you get a bunch of nothing. The case however contains about 12 boxes, so in those 12 there is often going to be at least one box that gives you some card of value.
The $500 box might have the potential to give you a $1,500 card at best, but at worst the best card you get may only be worth $40. The case of midrange boxes may only give the the potential for a $700 card at best, however because it's a case (and there's always at least one good hit per case) you know you'll get a card worth at least $150.