Can someone explain this code for me?

Question

What is the output of this code?
a=[x for in range(4)]
print(sum(a[1::-2]+a[::-3]))

Answer 1

There is a problem:
a = [x for in range(4)]
there is no variable before the "in", so this is a syntax error

Answer 2

Have a look at the corrected and commented source at https://code.sololearn.com/c9DA0Yd9E699

Answer 3

it was a question of a quiz. the given answer is 4. But I cant understand the mechanism