There were babies in the audience crying when we went. Some bits are quiet with the narrator talking, then loud thunder storms and bangs, flash lighting, loud orchestral music etc. The roars are very loud too. I would leave baby for a couple of hours with someone if possible. It's two hours including the interval, so probably a long time for baby to be held... may get bored. Also, your 4 year old may need on either Mum or Dad's knee at the loud/scary bits.
My 4 year old boy adored it, still talking about it a week later! We left our 2 year old son with his Nana, thinking it would be a bit much for him.