I have started my new job since may,200, previously i worked for the company for 2 years. Since, I have been working here for only four months,are this enough months? or should i ask my previous employer for the letter as well??'

See your other post.

http://www.visajourney.com/forums/index.php?showtopic=30955

I started my job in Sept 2005 and filed the k-1 petition in january 2006. I typed the paper for my supervisor to sign stating my hire date, position, salary, and full-time status. In your case, I suggest adding the dates from when you worked there before.