Is some BM really better than none?

Haribo1

I am a first time poster here and would greatly appreciate any advice.  I have been EP and supplementing with formula.  I have never been able to produce more than 6 oz total in a day, but I felt that some BM was better than none.  Now that I am back at work, my supply has been slowly dwindling.  I pump four times a day (7am, 12 pm, 4 pm and 9pm) for 20-25 minutes.  However, I am now only producing about 4 oz which is not even enough for a bottle.  I don't know if 4 oz makes any difference for her, and I should continue pumping, or should I finally just stop.  Does anyone know how much BM is beneficial?  TIA.