Source: ETS GMAT PAPER TEST 8

Q41:
When drive-ins were at the height of their popularity in the late 1950’s, some 4,000
existed in the United States, but today there are less than one-quarter that many.
A. there are less than one-quarter that many
B. there are fewer than one-quarter as many
C. there are fewer than one-quarter of that amount
D. the number is less than one-quarter the amount
E. it is less than one-quarter of that amount

My doubt is between the usage of fewer and less in the context of the above sentence.
Is fewer than one-quarter a correct quantity descriptor ?
Please provide some background on the correct usage.

OA is C.

Fewer is used for something that can be counted; less is used for something that is not countable. In this case, we can count an actual number, so we should use fewer.

How Can C be correct..
is 4000 not a number, .. how can it be an 'amount'?

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