dear son was a partial bf baby, we asked pdand was told that even babies who are on 100% FM dont need water during 1st 6mths, so partial and fully breast milk babies definitely dont need any too.
i started to give him water at 5.5mth old when he started semi solid. at 1st only give a few sips to let him get used to it and slowly increase the amount. now he is 18mth old, he love water to bits! will ask for water whenever he is thristy. as long as you only introduce PLAIN water (after he started semi solid) and not sweet drinks lik juice, he will slowly accept water.
there are many types of saying out there. depends on whether you prefer to follow the ways your relatives taught or advices given by doctors (pediatrician). for me, i chose to follow pediatrician's advices. anw your son is alrdy 4mth old, if relatives/family is stressing you, giving a sip or two of water is fine. But if you're talking about those NB babies, then i will tell you pls dont! =)