They are most likely pushing the narrower floor, because you region has wide humidity swings from summer to winter.

Summer the floor is going to swell and expand from higher humidity levels. If it is installed too tight for the maximum moisture content it will see, the floor may compress against each board, causing a cupped appearance. Too much swelling and it is going to buckle.

Now, install that floor acclimated in the summer at it greatest swelled state, so no cupping or buckling happens during the summers, come winter time and you crank up the heat, that floor is going to shrink and big gaps are going to form.

That is a lot of wood across a plank to shrink & swell, compared to a narrow strip floor.

Quarter sawn lumber if it is wide plank, helps reduce the shrink & swell.

If I were installing the floor I would prefer to do wider stuff. The wider it is the faster it can be installed. Their is absolutely no issues with installing 4"wide flooring.