D.W.
First, deep breath mom! Is your daughter growing and gaining weight at an appropriate pace? Has wet diapers and poopy ones at least every couple of days? Then she is getting what she needs!
By 4 months your breasts won't feel as full as they did in the early days because your production is stable. And you don't always feel the let-down by this point.
What you pump and what baby gets are very, very different things! Baby is much better and efficient at getting milk out than a pump will ever be.
Feed her on demand. That is the absolute best way to maintain your milk.
Don't watch the clock. Watch her. If she seems satisfied then she is. My daughter was a power nurser. She could nurse from both sides in less than 10 minutes sometimes! And she gained weight like a little champ and was growing and happy.
Breastfed babies are very adept at letting mom know what they want and need.
It does take more than a few days using the supplements but it really sounds like she is doing ok and you are getting stressed.
Again, follow her lead and let her nurse when she wants to. It is not uncommon to nurse more frequently at times. She could be beginning to teethe, she could just need more comfort, etc.
If you are still concerned check to see if there is a local la leche group, a lactation consultant at your local hospital, a birthing center with a breastfeeding educator or a breastfeeding support group where someone can come to you or you and baby go to them and let them see baby nurse.
I nurse mine until 16 months and 20 months respectively so I understand it's easy to stress but as long as she is wet/poopy/growing she is doing fine! Don't let anyone undermine your confidence in your ability to feed baby and finally:
Don't let anyone tell you you need to introduce cereal right now! That will not help your supply!