I have 3 boys & have been trying again for 2 yrs. I've had 3 miscarriages from aug '10 to june '11 & 1 earlier miscarriage. Any thoughts on why?

Lots of reasons. There are several possible reasons for pregnancy loss, from sperm problems that may not have existed before to older eggs to fibroid tumors that may be inside the uterus (and weren't there in the past). See a qualified fertility specialist who can test you and your partner and get to the bottom of your losses.
Seek evaluation. There are a variety of reasons on why one may have repeated miscarriages. Seeking an evaluation is reasonable after 2-3 miscarriages. Different causes may include genetic causes (chromosomal abnormalities), antiphospholipid syndrome, uterine abnormalities (e.g. Scarring), endocrine/hormonal abnormalities, clotting disorders, etc. ~50% of evaluations will yield a negative evaluation.