Bottom line guess to the question is, without knowing if your running +4 or -4 ....speedo reading +4 km/hr, possibly more on the odometer, speedo reading -4 km/hr, possibly less on the odometer.
I think that's the right answer.
The only thing we don't know is at what speed he's reading +4 Km high? If he's doing 100 KPH, then that's only a 4% error, but if he's going 50 and his speedometer reads 54, then that's an 8% error, and that would be pretty far off. Personally, I could live with a 4% error. The speedometer isn't a precision instrument anyway.
My Mazda reads a few KPH high at 100, and that's with the stock wheels and tires. That's compared with 2 completely different GPS units. For accuracy with GPS, you need to maintain a steady speed for a while, and the faster the better. 100 KPH seems pretty good for this. I've checked this against measured 5 KM stretches on the highway, and I'd say the GPS can easily get you within 1-2 Kph accuracy. Depends on your GPS, too, I guess.
I suspect the reason for the unavoidable speedo error may be that the Ford programming software for the PCM doesn't let the technician enter in a precise number for the tire diameter. It may just have a drop-down list of various sizes and he has to pick the closest one. Sort of like buying a pair of shoes -- your feet may be size 10.378245, but they only sell size 10 or 10 1/2.
I'm not sure about the PCM programming thing, since I've never seen it, so it's just a guess on my part. But it sounds pretty convincing, eh?