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The final drive ratio swap will improve highway FE, absolutely. Maybe a mile per gallon, likely more. Its effect on city driving, below 45 mph, is a LOT iffier. I wouldn't count on any gain there (you'd likely delay upshifts, that's about all) so the average engine speed can remain about the same. But if you do highway driving the most, you'd be seeing a real gain.
Of course, if you could recalibrate the shifts to work at the same vehicle speeds, then you'd be more likely to gain in city driving, too. I'm not sure how it's now calibrated -- to work off real vehicle speed or transmission output shaft (driveshaft) speed. If it's the latter than you'd definitely just delay upshifts with a lower numerical final drive.
Your four speed auto has an overdrive (ratio less than 1:1) in top gear. The somewhat higher FE you're seeing after being in 4th already, at a higher vehicle speed, is from the torque converter clutch locking up. That is DEFINITELY a good thing for FE.
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