I can imagine getting 4mpg better than EPA, since I've done so on my cars.....
but as a rule?
I don't think since someone 'claims' respective numbers, that a decision should be based upon that. Figure you'll get between 12-20mpg on EITHER engine, depending on the demands and strokes you're putting it thru.
No-one said the 4.6 is "BAD" (or if they did, were only using it for comparison, cuz it's a good engine too), but in the question of which is the "better" engine, most would lean toward the 5.4 ~~IF within the buyer's reach.

 

My 4.6 ain't even sniffing 18-19mpg. Of course only driving 3 miles to school, its probably only getting 5mpg due to the engine never getting warm. Makes me understand why many high school kids are driving Honda Civics, and almost makes me want to get one(sorry if that scares ya), but I'm working with a strict budget. But I thank God for it when I need to haul that deer out of a muddy place or pull my friends lil red Acura that took that sharp turn too fast.

 

The EPA numbers are a ratings system; they were never intended to be the ACTUAL mileage figures people would get. The numbers are to be used for comparisons with other vehicles. For example, if the 5.4 is RATED at 14/18, and a Dodge Ram is rated at 12/16, then you'd get 2 MPG better driving the Ford, REGARDLESS of how you drive it. If you find you're only getting 13 MPG with the 5.4, then you know you would have only gotten 11 MPG driving the Dodge. That 2 MPG difference is a constant.

 

Last August, my wife and I bought the 03 Eddie Bauer Expedition with the 4.6L for a simple reason that at the time the price was right, paying $35100 for an SUV that was MSRPing at $42990. It was perfect for mommy (with all the goody options which includes DVD, 1st/2nd row Captain's Chair etc....), but for me, after test driving an EB EXP with the 5.4, I wanted the bigger engine with the get-up-and-go. After driving an M3 for the past 4 years, I needed to feel the horsepower and torque. Like she said, “it’s my SUV I don’t need all that.” So I concurred with the purchase, but vowed in the future not to make that same mistake again.

Shortly after the purchase the new F150 started to arrive at the local dealer so I test drove one and caught the fever. After all these years of driving the Bimmer, I realized I needed to come out of the closet. I realized I was a truck man. I ended up trading in the M3 for 04 FX4 F150 4x4, but this time I GOT MY 5.4! Hoooah! Even though the Bimmer was much faster, I didn’t look back as I drove off in my awesome FX4 F150. GO FOR THE 5.4 AND GULP SOME GAS!

 

I test drove a 5.4 with 25 miles on it and felt a little disappointed. When I decided to buy my FX4 I bought the one in the showroom which has a 5.4 as well, but it had 500 miles on it before I even bought it. Let me say this - Dan is absolutely right. The FX4 I bought would run circles around the one I test drove. And they were identical trucks except for color. Ford must be manufacturing these engines to tight tolerances. Once they loosen up and the rings seat they show all their ponies.

 

I'm starting to wonder if the difference is due to a program in the computer, which allows the truck to break-in. Possibly advances timing after a certain number of miles. The difference is almost too much to attribute just to normal break-in. My snowmobile uses this break-in technology.