If you do want to start dropping pumping sessions and/or reducing supply, there are a few ways you can try:-
a) Lessen the time spent on a particular pump session (perhaps pick the most awkward daytime one but keep one evening/night session at the moment if you are still pumping then) dropping a minute or so daily until you don't pump at that time at all.
e.g. 12pm pumping is normally 25mins, so on the first day of dropping you pump for 23mins, 2nd day 21mins, 3rd 19mins, etc. If you find you coping well with that you can drop quicker or switch to method b).
b) Don't pump at that time but bring the pumping session before the dropped one later and bring the session afterwards back
e.g. you pump at 6am, 9am, 12pm, 3pm, 6pm, 9pm & 12am and you plan on dropping the 12pm session and you wanted to try this method you would bring the 9am session forwards to say 10am, and bring the 3pm session back to 2pm then you would be pumping 6am, 10am, 2pm, 6pm, 9pm and 12am.
c) Rather than dropping time spent pumping you could also drop mls pumped in a similar fashion (for the session to be dropped)
e.g. Rather than pumping 150mls a side (I'm going with big numbers here that I could only dream of as I was a rubbish pumper!), you try to pump 125mls a side for a day or so, then swap down to 100mls a side for a few days, then 75mls for a few days then 50mls (perhaps at that point you might try skipping the session altogether).
It really does depend on how you feel, what will work best for you and remember if you feel uncomfortable when dropping a session, you can always take it a bit slower or do a mix of methods (e.g. do option b but do a small/short pump at the normal pumping session time if you get overly full just to take the 'top' off).
If you go slowly, then if you notice a too big drop in supply it won't be too difficult to get supply back up. If you get a plugged duct while doing this then unfortunately all bets are off and pumping (pref. hand expressing if poss as you can concentrate on the plugged area) may well need to be increased just until the plug is cleared.
You can also try the suggestions on Kellymom.com for clearing plugged ducts here for other ways to try.
HTH and congratulations for pumping for 4 months!