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Hi

Its worth checking with the airline as they differ.
Some of the budget airlines you have to include them as your luggage allowance. But people like BA and Virgin carry them as extra, and they are free. You will be able to drop the buggys off just as you board the plane. But warning this does not work in reverse, and you will have to collect it with the rest of your luggage. No buggys are allowed in the cabin. So a sling/carrier might be useful.

Good luck

Pushchairs are great if you have someone else to help you but if you're travelling alone, or if all the other people have their hands full, you'd be better off with just a carrier so your hands are free.

Why a double buggy, then? Does one of the toddlers need it as well as the 4 month old? As has been said, check with the airline. The cheaper your tickets, the more likely they'll 'do' you for the buggy as extra baggage (thinks 'Ryanair'). Do you have a sling? I honestly think it would be invaluable, if you've got enough people willing to share the burden (not that 4 month olds weigh that much), a sling would mean you probably wouldn't need a buggy at all.