Answer
We can rank the detectors according to the frequency of the sound they detect from the source:
$1 \gt 4 \gt 3 \gt 2$
Work Step by Step
The source moves directly toward detector 1, so detector 1 will detect the highest frequency.
The source moves directly away from detector 2, so detector 2 will detect the lowest frequency.
The source moves parallel detector 3, so detector 3 will detect the original frequency.
A component of the source's motion is directed toward detector 4, so detector 4 will detect a higher frequency than the original frequency, but a lower frequency than the frequency detected by detector 1.
We can rank the detectors according to the frequency of the sound they detect from the source:
$1 \gt 4 \gt 3 \gt 2$