I ran 4 miles earlier in the week and now my lower back has been hurting could it be a strain or a pulled muscle?

Back Strain: Low back pain is a fact of life. Just about everybody will suffer from it sooner or later. One of the main causes of back pain, whether acute or chronic, is low back strain. http://www.webmd.com/back-pain/guide/low-back-strain

Actually, it is!: The low back is stressed with almost all activities. The muscles help keep us erect with standing, running, sports, sitting...essentially everything. This can be associated with tight hamstrings or vigorous exercise greater than is typical for the individual. Treatment is ice, heat, hamstring stretches, hydrate and massage. Take meds only if necessary. Remember to warm up and stretch afterwards.