My hydrostatic Hondas can travel faster than I can comfortably walk. But jogging while mowing? I wish my lawns were that flat and smooth.
A small adjustment to the transmission's cable up near the handle will produce relatively large changes "on the road."
Meaning: the mower can go out of adjustment, and might need a simple adjustment.
But we don't yet know if yours is already achieving the 4 mph top speed and you're asking for more.