Pain in the lower abdomen is very common in this month. Even I am having such pains nowadays since I am also in 4th month. I came to know that the reason for the pain is, our body gets prepared for the normal child birth where sometimes we feel pain in the lower abdomen because of the hormones. these hormones dilates the vessels and make the pathway loose.
So there is no need to worry about the lower abdomen pain since its common unless if you get more severe pain frequently you have to visit your doctor.