hard to say on that basis. have you spoken to him at all for an extended
period of time? People can cope with quite a high level of deterioration as
long as their circumstances dont change. OTOH its unusual he hasnt
(apparently)  deteriorated at all but then again unless you interact with
him you may not be able to tell. 58 is also quite young to get it (though it
does happen)

There are many causes of so called "dementia" that often are labeled
Alzheimers, though they may be from other, treatable causes.   Possibly his
diagnosis was faulty labeling, and whatever he has that caused some
cognitive troubles was treated already?   There are dementias that are
caused by mini strokes, or by circulatory problems, and even from certain
thyroid conditions, and they are not alzheimers, though they might be at
first mistaken for it.

Only his family and his doctor know what his level of functioning is.   If
he is cognitive enough to know how to cut the grass, that is pretty good for
someone diagnosed four years ago.   Same about the driving and watching a
baby.

For instance.... my mother in law was diagnosed 4 years ago and she is now
incontinent and in a wheel chair and unable to function without skilled
nursing care 24/7.

Not knowing the man, his level of functioning, or what caused his doctors to
diagnose him with alzheimers, it is impossible to judge.

I would say that if it really and truly was alzheimers, he wouldn't be
functioning this well four years down the road, but that is just an
uneducated guess.