G.K.
The only thing you can do to decrease milk supply is to decrease breast stimulation. 2-3 hours is very typical for a 4mo to feed, so your body is just staying on the schedule you set for it. When you start to feel uncomfortable, try expressing just enough to decrease the discomfort instead of doing a full pumping session. The other thing you can try is to slowly decrease your pumping time. If you're currently pumping 15 minutes, decrease to 14 minutes for a few days, then down to maybe 12 for a few days until your comfortably producing the same amount of milk at 10 minutes. 10 minutes is about the recommended time to pump. This will also help your body change its schedule and amount of milk per session. Also, keep in mind that research has shown that babies take an average of 25oz from 1-6mo. So, it's not that older babies need more milk; they nurse less often but get more milk per feeding. There's a really cool milk calculator here: http://www.kellymom.com/bf/pumping/milkcalc.html. Great site, too.
I can also personally recommend http://www.nursingmothers.org. I'm a peer breastfeeding counselor with Nursing Mothers Counsel (in the SF area). We help moms all over the country through free, one-on-one phone counseling, but our website is a great resource as well.