There could be a couple of things going on. First off, as you approach the 4 month mark, your body changes how it produces milk (some interesting reading here:
http://kellymom.com/pregnancy/bf-prep/milkproduction/) from being more hormonally driven to supply and demand driven. Our bodies are told to make milk when the breast is emptied. 4 months is a point when a lot of women feel like they are "emptier" so what you're experiencing could be related to that.
The best judge to see if your supply is ok is your LO -- if she's gaining well, getting enough wet/poopy diapers, seeming content between feedings, meeting milestones, etc, chances are that your supply is totally ok.
You can do a few things as you transition -- you can feed every 3 hours and leave your A/S times on more of a 4 hour -- you'll probably end up with two feeds closer/further apart because of the timing,which is ok. You can also feed at the beginning of your EASY cycle and then do a top up feed about an hour later as well -- this will keep you on a more "traditional" 4 hour cycle, but still get more feeds in.