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Yea, 4000rpm is pretty high. If you ever look at the few BSFC charts out there you'll notice light vehicle engines tend to start losing efficiency above 3000rpm at any load, presumably because friction is getting pretty serious. For short geared cars people tend to report pretty big mpg changes with taller gears because of this; my MR2 runs a little under 4400rpm at 80mph (no, I never cruise at 80, I do 55 and let people blow past me at 70), people who swap the 5th gear to an 11% taller one have claimed pretty big improvement (like 9%ish) when doing 70mph.
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