And what about the following case:
0 0 0 1
Or billboard's leg can't be on the road? If it can, we see:
ang(0) = 0;
lim[ang(x)] = pi/2 (x -> 0). So, there is no best angle.

Yeah... It seems to me, i should output pi in this case.

Yes, answer is PI, like in sample #4

Correct answer is PI/2 when x1=x2 and y1*y2=0. But there is no such test case awhile.