http://www.bflrc.com/newman/articles.htm
http://www.kellymom.com/parenting/sleep/4mo-sleep.html
Hopefully the two links above will help you get some reliable breastfeeding information. I don't have a lot of time to write, but I agree with other response - your supply is probably not going down... just adjusting. Or it may be a temporary decrease due to your period. Did you just start your baby on solids? Babies should now wait until six months old to begin solids. The reason I ask is because women tend to start their periods around milestones such as baby starting solids or baby sleeping through the night or baby walking. (for me it was walking.)
But the article on wakeful 4 month olds is EXACTLY what happened to me. But at that time, I was only giving bottles while I was at work. The best way to induce supply is to have the baby nurse at the breast, in my opinion... and that's based on what I've heard and read. Are you exclusively pumping? There are helpful message boards on kellymom.com for moms who are pumping or inducing lactation. Also try that resource or a local lactation consultant if you can. Try a non-profit hospital... I never had to pay for a consultation through a non-profit.
Good luck to you though and stick with it!! Four months old is a big developmental stage starting as the above article describes. Also...the jack newman articles have a ton of breastfeeding topics and myths you can also read about it. Those resouces have been lifesavers for me!!