My lactation consultant told me that she did her master's theses on SIDS, and other countries have done a lot more research on the causes than the US has. A scientist in New Zealand has done a lot of research on SIDS, and he has come to the determination that it's a combination of the flame retardents in the mattress, moisture from the baby's breath, and mold that is naturally present in all mattresses that causes a toxic gas to accumulate at the surface of the mattress, which causes the baby to stop breathing. I purchased the Babe Safe mattress cover from the website below, and we haven't had any problems. It's about $30 (or was when I ordered mine). I think this theory makes sense, as I've read recent studies that say babies that sleep in a room with a ceiling fan are less susceptible to SIDS - the fan keeps the air circulating and prevents the gas buildup near the baby's face. That's also why they've found a higher instance of SIDS with babies who sleep on used mattresses. An older mattress means more mold.
http://www.cotlife2000.com/
I never put my daughter to sleep on her stomach, but once she could roll herself over I left her on her stomach if that was how she wanted to sleep. Unless you're going to stand over Dylan's crib the entire night, you've got to let him sleep in the most comfortable position for him. But since he's only 4 months old, I'd really keep putting him to sleep on his back until he's old enough to roll himself over.