Are you sure those are the right criteria? Considering the fact that CAFE (Corporate Average Fuel Economy) requirements continue to increase, fuel-efficient today and long-lasting (i.e., fuel-efficient tomorrow) might be at odds with each other. Maybe what you really want is lowest-possible total cost of ownership instead (which would change the recommendations I make below)?
Anyway, back to the original question: fuel efficient and long lasting.
You say it needs to seat four people, so that (very unfortunately) eliminates the Honda CRX, first-gen Insight, and the Geo Metro XFI (which would otherwise be the clear winners).
Priuses and Leafs are certainly fuel-efficient, but those batteries will fail eventually. I honestly don't know how big a problem that really is...
The other option, of course, is biodiesel. An old Mercedes 300TD is not fuel-efficient by modern small-car standards (although it's not bad for its size and weight), but there is no longer-lasting car. A somewhat-newer fourth generation VW Jetta or Golf TDI will get almost-Prius-like fuel economy, with an engine good for half a million miles, easy (but you can't necessarily say the same for other parts fo the car).
Finally, there are always the generic plain-old-gasoline-engine economy sedans. IMO, it doesn't really matter at this point which one you get; anything from Hyundai, Honda, Toyota, Nissan, Mazda, Kia, etc. will have pretty good fuel economy and pretty good reliability. (I am not at all convinced that, after 2000 or so, Hondas and Toyotas are any more reliable than the other brands listed.)