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I'm quite surprised as well and wouldn't mind someone checking to make sure I'm not an idiot reading the chart wrong even after quintuple checking.
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I'm floored by that stat. And not that it approaches 4% of a win (obviously), but there is (I'm guessing) a much greater than 4% chance that a save situation will arise tomorrow.
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The main thing you are missing is the difference between P(Rivera saves) and P(Proctor saves).
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To the extent that I'm missing it, it's because no one (me included) has produced it. Also, it's unlikely to be the main thing, in that we aren't talking about that situation...
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It is absolutely the main thing, because you claim that it is optimal to save Rivera for a "better" situation, which you maintain is bringing him in for a save situation either later in the game or tomorrow.
The probabilities we need to compare are
P(Rivera gets out of jam in 9th) * P(save situation comes up later in the game or tomorrow) * P(Proctor converts save)
and
P(Proctor gets out of jam in 9th) * P(save situation comes up later in the game or tomorrow) * P(Rivera converts save).
According to the figures you made up, P(Rivera gets out of jam) = .33 and P(Proctor gets out of gam in 9th) = .25.
According to the Hardball Times article I linked to earlier, P(average MLB ace saves a 1-run, 1-inning lead) = .81. Proctor's a pretty good reliever, and there are some crappy "aces" out there, so let's say his ability to save a 1-run lead is close to this.
Rivera's
career save percentage is .88, and while that includes some tough multi-inning saves, it also includes a lot of cushy 2- and 3-run, 1-inning saves. I'd estimate his ability to save a 1-run 1-inning game at around .85.
We have
P(Proctor gets out of jam) * P(Rivera converts 1-run 1-inning save) = .25 * .85 = .2125
whereas
P(Rivera gets out of jam) * P(Proctor converts 1-run 1-inning save) = .33 * .81 = .2673,
ergo, it is better to use Rivera to put out the fire in the 9th! The situation is even clearer than this, because the save situation that materializes down the road may be an even easier one in which the difference between Proctor's ability to save a lead and Rivera's ability to save the lead is even smaller.