（1）Computers are important research tools.
（2）Computers are an important research tool.
We can say that the first sentence treats computers as discrete objects.
The second sentence treats computers as a class or set of objects.
（3）Horses are useful animals.
（4）Horses are a useful animal.
We can say that the third sentence treats horses as discrete objects.
Why can't we say the fourth sentence treats horses as a class or set of objects?