Interference of light waves

The first thing you need to do is make an assumption about the relative phase of the waves emitted by each source which with no other information you make zero.

Then you need to add the amplitudes from all four sources taking account of the relative phase due to the distances the waves have travelled and then square the result to get the intensity.

The answer should be zero since each source is emitting light of same wavelength and same intensity thus same amplitude.
The path difference between $S_4$ and $S_3$ is $10.5lambda$ thus their phase differnce is $pi$ and hence they interfere destructively giving 0 amplitude.
Same is the case for source $S_2$ and $S_1$ which gives zero amplitude.
Now net amplitude is $0+0=0$ and hence giving zero intensity as intensity is directly proportional to square of amplitude.
This answer is applicable if sources emit light in same phase.