As the others have stated, modern vehicles with 4-cylinder engines have sufficient power for acceleration onto a highway. And, once you are up to speed, they will get better mileage than a 6-cylinder engine.
That being said, there is also the noise factor to consider. When accelerating, most 4-cylinder engines have to work much harder than a 6-cylinder engine, thus making them much noisier on acceleration. That is one of the reasons why I opted for a 6-cylinder engine in my present car. I am willing to pay the 1 mpg “penalty” for the smoothness and quiet of the six.
And, if you find a Ford Fusion being sold in the US market with a V-4 engine, please post back with information on that unique car!
You should be aware that the 4-cylinder engines currently being marketed in this country are either I-4 or H-4 engines (the Fusion has an I-4). The sixes that are on the market include V-6, H-6, and I-6 designs, with the Fusion utilizing a V-6.
If you tell a car salesman that you want a V-4 Fusion, that will mark you as someone who they can take advantage of, due to a lack of knowledge. Try to be as knowledgeable as possible when dealing with a car salesman, and you are less likely to be over-charged.