J.
The BEST resources I've found are www.kellymom.com and La Leche League...at 4 months (my daughter is about 3.5 mo) they're probably eating several times a day, probably every 3 hrs except for a 4-5 hr "sleep" period...you will need to pump that often...the BEST thing you can do is pump a little extra and bottle it...then STAY IN BED WITH HIM for around 3 days doing nothing but nurse, nurse, nurse...this will build your supply back up (the more he nurses, the more you make) ALSO, keep in mind YOUR BODY DOES NOT RUN OUT OF MILK! That's an old wives' tale...once the baby has drunk the capacity immediately available in your breasts, your body will make more WHILE baby is nursing so only use the pumped milk if baby gets upset...
You can definately rebuild your supply and begin breastfeeding again, the key is demand nursing...nurse ANY time the baby exhibits signs of hunger...
GOOD LUCK!