The reason I asked why the OP was unsatisfied with 4e was that he expressed that he had enjoyed material from the Book of Nine Swords. Since this was an early playtest of 4e material, and from my experience many of those that don't like 4e also didn't like B9S, I was curious what the issue was.
I am not sure that either system is superior for a "sandbox" game. I like that it takes less time to run a game in 4e, and it seems like it is a lot easier to "go with the flow" in 4e. I am not really sure what the term "sandbox" means. I have stepped into the RPG theory morass before without knowing what terms mean, so I guess I should ask what the OP means by "sandbox". How does 4e limit this?
As an aside, I am not out to set anyone up to be ambushed by this question. I like 4e for what it is. I also think that 4e could be better. I liked 3.x fine enough, especially in my houseruled campaign that utilized psionics for magic and B9S for warriors. Much better than core. No CoDzilla or McGuyver wizards in my campaign. No vancian casters. Fighters sucked, but it didn't matter because no one played them. 4e is not perfect for my campaign either, but I can still houserule it until it is, and the underlying assumptions are easier for me to handle as a GM. Easy is good in my life. Just wanted to know what the OP was thinking.
EDIT: Also, are you sure that your dissatisfaction with 4e is not a dissatisfaction with the DMs ability to run a "sandbox" game? Maybe you just prefer the narrative style that you had before. Maybe the DM just hasn't gotten his legs. I am not sure, and I am not really trying to convince you, but in medicine, you always have a diffferential diagnosis for the problem, to make sure that you aren't overlooking something. The same process could be utilized here to good effect.