My 6-week old daughter has not had a bowel movement in four days. Is that ok? She is both breastfed and formula-fed

Could Be. Breast fed babies can poo a little every feed or overflow a diaper once every 7-10 days. If soft it is normal. Straining or grunting sounds also do not mean constipation.If baby puts out rabbit pellets or shot cigars they are extracting too much water from their poo. Occasionally the combination of formula + bm can lead to irregularity.
Probably. You can try feeding her a bottle of apple juice and see if it induces her to move, but the fact that she is breast fed often makes infants not need to move their bowels for several days. This is related to the fact that breast milk is almost completely absorbed, which does not leave much to make a bowel movement. This is why you cannot judge constipation on frequency.