In answering a question on applying old-school rules to 4E, R.S. Conley suggests "Not use the encounter system. Just set up a region sandbox style and what the player find they have to deal with"
I don't understand what exactly the encounter system is, then: I thought it was the way to setup each encounter: "an encounter should have X number of enemies, according to the level of the party, adjusting up or down for difficulty", wherever it happened, not blocking a sandbox encounter; and not a fixed "here there should be an encounter, and here another", as I understand the answer. Since I have also seen at least one successful sandbox setup, I think that my interpretation of "encounter system" must have some flaws.