Hold off until your baby girl is closer to 6 months. Your son will want to love her to death.
My 2 are about the same age difference and I also have an older boy, baby girl. They sleep in separate rooms because their rooms aren't big enough, but I did want them to share.
However, when we've gone away on holidays they have shared a room and it hasn't been a problem. My ds is a heavy sleeper and dd doesn't wake him when she cries out for a feed.
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