I wouldn't be giving probiotics to a 4 month old, because I think that your breast milk shoud be probiotic enough! While it doesn't contain probiotics per se, regular feeding will expose him to the skin of your breast and nipple and breast milk does contain a gut protecting substance that will help repair the gut and return some balance to his digestive flora.

In the mean time, lots of good nappy cream at every change - chemists sell a boric acid / zinc and olive oil cream called BOZ which is good, and we use Weleda nappy change cream which has calendula in which is excellent, and I would highly recommend.