Clearly, whole numbers specify how many elements there are in a collection while decimal numbers specify how much of a substance there is in a lump---but only after a unit of that substance has been chosen. But then whether we use a whole number or a decimal number depends on the chosen unit:
0.004 Kilometers is a whole number of meters, namely 4.
0.00004 Kilometers is a whole number of centimeters namely 4,
But it gets worse: While
0.00004 KiloDollars is a whole number of centiDollars, namely 4?
can we really say that
0.004 KiloPeople is a whole number of Peoples, namely 4,
But then what about
0.00004 KiloPeople is a whole number of centiPeople, namely 4?
Where do you draw the line between whole and decimal and how do you explain it to very raw beginning students who want to understand? (Saying that, here, 4 is really the decimal number 4.0 does not really help.)