[Maths olympiad question]Why is the answer D?

You have assumed that numbers in the first row will also be same which is not the case nor mentioned here.
Given is only the numbers in the second row are same so let the number be a.
So, the third row now will contain numbers 2a and 2a and finally the final row will contain 4a. From that you can see the answer must be D.
It doesn’t say that the top row can be *any* multiple of 4 — just that it *is* a multiple of 4. Since every multiple of 8 is also a multiple of 4, D is always true by default.
Just to elaborate,multiples of 8 are a subset of multiples of 4.But not all multiples of 4 (eg 12) are multiples of 8 which is why I don't get why D is the answer
If the numbers in the bottom row are a, b, c, and d, the numbers in the second row must  be a+b, b+c, and c+d.

The third row numbers would be a+2b+c and b+2c+d.

The top block would contain a+3b+3c+d.

But the second row numbers are given to be equal, therefore a+b=b+c, so a = c; also b+c=c+d so b=d.

Substitute into the top block expression to get a+3b+3c+ b or 4b+4c, which must be a multiple of 4.
This seems way too easy to be an Olympiad question