My 4 month baby has been really fussy the last few days and arches her back and cries during feedings. Its like she wants to eat but sonething hurts?

Acid reflux. Your baby could have acid reflux (also called gastroesophageal reflux) when stomach contents refluxes up into esophagus and can cause burning or pain. Reflux can be made worse by formula intolerance, constipation, and illness. A 4 month old with fussiness that persists for a few days should be seen by her primary care provider to rule-out more serious problems.
Needs to be seen. She needs to be evaluated by her pediatrician. Increased fussiness and poor feeding in infants can be a sign of infection, reflux, milk protein intolerance, and other causes.
Possibly reflux. As long as your child is healthy, and has no ill sx. This is probably reflux. If bottle fead, you may initially thicken the bottle with rice cereal 1/2 to 1 tsp of rice per formula made. If not helping then your dr. May consider treating with an antacid or ppi (medications for reflux).