Aiyoh, I feel you lah, a few of us mums faced the same thing and it's super stressful when baby refuses formula. Since your milk supply is low, maybe try mixing a bit of the formula with your breastmilk first (even just a few mls) so baby gets used to the taste, or warm the bottle a bit warmer than usual, sometimes babies are picky about temperature. But honestly, at 4 months with low milk supply AND baby rejecting formula, this is something your doctor should check properly, because baby needs to be eating enough to grow; they'll also rule out any latch issues or tongue-tie that might be making feeding harder. Don't stress yourself more, just get checked out so you have a real plan forward, okay?