My pediatrician says 4 days of no poop is okay..even 6 days. But if you're really concerned or something seems off, call your pediatrician.
Some other things you can do to help him out include giving him a small amount of diluted prune juice. At six months he could try to begin eating some purees, give him some fruit like apple and perhaps simple veggies like carrot. Avoid pears and bananas, they are binding. Broccoli and Cauliflower cause gas and bloating. He won't need much to get things moving. Avoid suppositories. They cause more trouble than good.
As for yourself, since you're breastfeeding, you probably already know what you eat can affect baby too. Take inventory of what you've been eating too. If you've eaten alot of dairy or other foods that are binding, too spicy or gaseous, your breastmilk will affect him too.
Last but not least, a warm wash cloth on the tummy or any sort of mildly warm compress on his tummy will give him some relief as well.
Dr. Sears has a great book titled "The Fussy Baby Book" and while your son isn't necessarily fussy, he may benefit from the chapter that highlights several "tummy soothing" massage techniques and positions that a parent can use when holding or trying to soothe baby. Sears says they're not only excellent for babies with GIRD, but for children who might be constipated or have mild tummy discomfort. You might find some of these online too...or try searching for words such as "the clutch hold" or "colic and hold positions".
UPDATE: HERE'S A SITE I JUST FOUND THAT EXPLAINS SOME OF THESE HOLDS.
He might be gassy too, so perhaps gas drops.
Hope some of these ideas help.