With 4 wheels, you will be pumping the grease gun hundreds of times to purge the old grease out of the bearing. If you have carpal tunnel or weak hands, the pistol style will cause hand cramping.
If you can have a helper slowly turn the wheel while you pump, I highly recommend a lever gun that can be placed on a firm surface and displaces the pumping energy over your entire arm and upper body. Per the manufacturer, the lever style also displaces more grease per stroke, reducing your effort. I have used the Lock n Lube lever model with great success, the locking zerk fitting makes a big difference.
Standard pistol style
or lever pump style...
