sensitivities can show up immediately or could take hours. my concern would be that you can easily compromise your breastfeeding relationship with just two ounces of formula. if baby seems hungry, it is probably the four month growth spurt and they are just working to up your supply to meet their needs. as long as there are plenty of wet/poopy diapers you should not be supplementing. your body can make what your baby needs, don't doubt that. the percentage of women who cannot meet the needs of their baby is miniscule and the odds of you being one of them are very small. if you have to worry about something like that and watch for a reaction in your infant, is it really something you want to give her?