Go to a WalMart/Target and buy the Cosco Touriva and pack it with you. It's $40, it's lightweight, and rates well with Consumer Reports. If you have a purchased a seat for your infant (or there's a light load) you can use it on the plane, if not you can check it in without paying extra as it does not count against the baggage allowance. And do use it on taxis--all the time (some of the worst drivers are taxi drivers). If you're renting a car for 4 days, you've paid for it. Back at home use it on a second car or loan it to friends. You will also use it on planes after your kid is 2, as by then you'll have to pay for the seat anyway and the seat is still a recommendation (should be a requirement, IMHO) until the kid reaches 40-44 lbs./20kg.

Buying the Cosco Touriva has been by far the best travel-related decision I made as a parent.

Oh, yeah, in cars always secure a car seat using the seat belt and not the 2 LATCH snaps: Consumer Reports testing made it clear that the LATCH system sacrifies safety for convenience (do anchor it, though).