What can i do if my 4 1/2 month baby refusrs to drink liquids because vomiting?

What liquid?: At 4 1/2 month: both breast milk and appropriately mixed formular contain adequate amount of water, thus additional water is not really needed nor advised. If he/she is sick and not able to breastfeed or take in formula, then some premixed fluid such as pedialyte can be tried..... Anyway, if your baby appears ill, he/she should be seen by doc asap...