E.T.
Oh yes - I've been there. If she's suddenly started not eating as much and being fussier (and throwing up more), then she may have outgrown the Zantac doseage. It goes by weight. Or she may need a stronger medicine. Zantac doesn't work for something like 80% of babies with reflux. They need Prevacid or Prilosec. Talk to your pedi about trying one of the other medicines. As for rice cereal... it's a double edged sword. Yes, she'll be less likely to spit up. But she'll be getting less nutrition since she'll have fewer ounces of formula and more empty calories (the rice has virtually no nutrition). Plus, since she's only four months old, her system is probably not ready to digest solid food and the rice cereal may make her backed up, gassy, and ultimately lead to more reflux.
What worked for us was a combination of Prilosec, Nutramigen, and making sure that my daughter was fully upright (ie - held) for 30 minutes after every feeding, plus raising the head of her crib so that she never slept flat.
The spit up is super annoying, but it's actually the least worrisome part of reflux. Many doctors refer to spitting up as "a laundry problem" not a medical problem. So if you can adjust how she's sleeping (not flat) and get different medicine, you may find she's eating well, not fussy, and sleeping ok. If so, I'd just deal with the spit up and avoid doing the rice cereal. My two cents.