There is no issue with a 4 month old being at a wedding reception. I would speak to the bride directly, it could have been said in passing at a "bridezilla" moment, especially since you are making the effort and travelling interstate.

When we got married earlier this year it was a definate no children for us at the reception but we considered that from about 2 years & up.

My DH's rellies had to travel from interstate one with an 18 month old and the other with a 12 week old and there was no way that I would have even considered saying they had to pay a sitter or couldn't bring them to the reception.