I would not be leery of the engine at all based on some 5 year time frame. The primary things that would be affected because of the sedentary life this machine has been subjected to are the engine oil, the battery, and the fuel system.
Having said that, you should insist that the oil be changed (and filter) before you take delivery.
If the battery is the original one, you definitely want them to replace it before you take delivery.
You also want them to install a new fuel filter.
The above recommendations (mine) are due to the following concerns.
4 hours of run time over a 5 year period is about right for starting and moving periodically as needed. But I doubt the engine has really had a good warm up to dry out the moisture that gets into the oil on a cold start. So you want that oil, and oil filter to be changed before you put it into service.
If the battery is the original (it may not be) I presume it has run down a few times in 5 years, requiring a boost start and/or recharge. That kind of treatment is "deadly" to a battery and if not replaced, you may expect a short life of the battery.
The fuel filter should be replaced because the gasoline it has been filtering has likely been "stale" most of the 5 year period which would coat the filter media with varnish and other forms of debris that can occur when stale gas is present.
Do your research for the average cost of the above parts (battery, oil and oil filter, fuel filter) before you begin negotiating for the lowest price they will take.
If they are hesitant to replace the items, ask for a further reduction in the cost of the machine that would reflect what you would have to spend to have the work done.
If they refuse to bargain with you on those issues, maybe their stubbornness is why they have a new 2006 Husqvarna in stock.

Any mower that has been sitting around with old fuel will probably have problems. Since the 2006 mower at the hardware store is moved every day, the fuel system is probably ok. The battery is nearly 5 years old and may need replacement. If the mower was parked outside, rain water could have possibly gotten into the spindle bearings. The seat may have received some sun damage. I would evaluate these items. A garden tractor usually has a 46 - 54 inch deck. Make sure you need a large deck by considering the size of your yard. Some of these used the Briggs Intek engine and some are equipped with the Briggs Vanguard. The vanguard is a more durable engine, but occasionally carburetor problems develop. The Intek sometimes has a problem with old fuel and sticking intake valves. These problems are easy to repair. Most people never have a problem with either engine. Spare parts for the tractor and the engine are easy to find through a mower shop.

I believe the JD 140 is equipped with the Briggs Intek and again, old fuel may cause a sticking intake valve. Spare parts are readily available through the John
Deere dealers.

Many of the Sears craftsman tractors are built by the Husqvarna Group but simply have a different paint job. If you look in Sears, check the model number that is under the seat. Models beginning with "917" are the Husqvarna. Lower priced craftsman models have a 247 model numbers and these are not as durable as the Husqvarna built tractors.

I don't know if Kawasaki is offered on the tractors you mentioned. This is a very durable engine that I believe has fewer problems than the other engine brands. Spare parts such as air filters are not quite as common in the mower shops, but can be ordered and received in a few days.

Most of the problems that develop in any small engine are usually related to old fuel and lack of regular service. For lawnmowers, fuel is considered "old" if it has been sitting in your tractor or your spare gas can for only a few months.