Well, I am not a commercial user so consider that in my reply.
In my estimation the HD-4 is a very tough machine. Mine happens to be a 4 cylinder diesel unit and it has a lot of power to push and lift. Great hydraulics even though it is a relatively low pressure system, 1,800 PSI I think. It has a loader with a large bucket but I have used it for digging and grading as well. I like the shuttle shift as there is no clutch and you can go forward and back by just shifting a lever. Functionally it is well built. That is the up side.
The down side for me has been the difficulty in working on it. The factory built these from the chassis-up with no apparent regard for maintenance. Meaning that access to lower components is terrible. I had the main hydraulic pressure hose blow and it was a two day job to replace it since it was so buried under other hoses, linkages, and floorboards. Same for brakes and steering clutches as the fuel tank and hydraulic tank cover access to the inspection plates. Same for the radiator. If it has a loader the radiator is extremely difficult to get to and requires lots of time to swap out. Bottom rollers require a huge effort to replace as both rock guards have to be removed to replace any one of them.
Because of age and the difficulty in working on these machines, I would consider mine to be scrap if anything major ever failed.
So, if you have an eye on a well maintained one for a good price, it probably would be a real workhorse for you. I would much prefer the diesel model. Avoid working on one if you possibly can.