DOM,

A 4 month old will be fine for a flight, our youngest has been 2 months.

As explained already you can't buy the infant ticket until the infant is born. You can, however, put in the request for bulkhead seats for use of a skycot as soon as you have your tickets. Our last two family vacations were with Virgin, and our youngest was two months and then fourteen months old on these trips. On all flights got the bulkhead seats - but that doesn't then mean that it will have the skycot fitting. Each time, where it was appropriate, the cabin crew did a wee re-jig of other bulhead passengers across the row with us so that we could get the skycot fitting.

Would agree with Julie about the size of the skycot, but BA also have a Britax made car seat like thing, which is better for older children.

If I can suggest the best thing is if the baby can breast feed as then you don't have worry about sterilisation or perhaps supply of feed (just make sure mother doesn't get dehydrated!) plus it means that the baby can easily feed during take-off and descent (more tricky as it starts 30 mins from landing).

Best advice is to try to chill, don't try to plan everything and roll with the punches.