I flew from Sevilla, Spain to Portland....on 3 legs...my daughter was 6 months. We were coming back from a wedding and she got a cold, my husband had a cold and I got the same cold on the flights back. It was about the worst experience I have EVER had in my life. Sorry to say, but it is the most painful thing to have your head full of snot and pressurizing in the air. It feels like your head is going to explode and you can't hear. It is awful. Also, you want your baby to be sucking in some fashion (whether it is breastfeeding, sucking on a pacifier, etc.) as you ascend and descend, for their ears. If they can't breathe when they suck, then it is almost impossible to have them continuously suck. We were going to fly straight home from Sevilla, but after we landed in Miami, we changed our ticket (because the airlines were very accomodating due to major illness of an infant), and we stayed there overnight in a hotel in the airport. We spent way too much extra money in the end, but having that extra night without changing so many times about saved my sanity a little. I was out $200 for the hotel room. But looking back it was a worthwhile $200. So, in short (or long by now) if you can change the vacation, or delay in any way, I HIGHLY SUGGEST IT. Because infants that are 4 months are much more suseptible to diseases and when they have a cold their immue system is compromised. I wouldn't take the chance of a trip from _ell...and putting your child in jeopardy of getting a worse illness. Shots don't prevent every kind of terrible disease, like pnemonia. Shots only prevent perhaps a strand of that disesase anyway. Anyhoo....good luck to you, but there's my 2 cents.