From what I can tell, the main difference between the HRX217HZA models is that the K3 comes with an external battery charger and the mower does NOT charge the battery while running, while the K4 has the auto-charging circuitry, and the external battery charger is available as an optional accessory. I believe the battery is also a bit more easily accessible on the K3, with a clip on battery cover since the battery would need to be accessed more often to attach the external charger, whereas the K4 battery cover is secured by three screws.
I compared a replacement part list, and the K4 has a different magneto coil, presumably for the additional charging circuit. The transmission and everything else appears to be the same on each model, and cosmetics and mower functions appear identical.
The local Honda dealer tells me that the K4 was generally designed for climates with longer mowing seasons and less storage time which would tend to keep the battery charged, where the K3 is for areas with longer storage intervals which would deplete the battery charge and make external charging more desirable.
Hope this helps...
Ron S.