My wife and I did sex on 8th, 9th of August,2013 and 11th she had periods.
Now next cycle is delayed by 4 days from 11th September
Is it possible that she may pregnant?

You had sex in August, she got her period in September. I would say, good chance no. Might just be a wonky period. It happens. If it doesn't come in another 10 days, take a home pregnancy test.