4E is actually built to give the PCs probably like a 90% success rate against a standard encounter build
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And when the PCs in a 4E game don't... it usually will be due to the DM purposely putting them up against an encounter built specifically to be more of a threat (one meant for a party like 4 levels higher or so).
This is an interesting line of thought.And whereas the unpredictability in pre-4E usually was because of swinginess... the unpredictability in 4E usually is because of DM encounter building. And it appears as though some people just prefer that the dice fall where they may, as opposed to leaving it up to the DM to dictate that unpredictablility.
For me, the unpredicatability in 4e is primarily in the way an encounter unfolds, and the things that are done (by both PCs and NPCs/monsters) on the way through. The outcome is, in a sense, simply the sum of those things.
By scaling the difficulty up or down I (as GM) can influence how things will unfold, and I can certainly increase the pressure, and hence the urgency and care with which the players make decisions. But ultimately the unpredictability comes from their decisions, as mediated via the action resolution rules (including dice rolls) and that is independent of the degree of difficulty.
A simple example from my session on the weekend is forced movement + cliffs: the unpredictability here results from the saving throw to fall prone at the edge of the cliff, and then - if a PC does fall - how s/he responds to that. And that is not really a function of encounter difficulty.