There really are no other options for your one year old. Are you renting a car at your destination? You could always rent a car seat from the rental car company. If you rent from Hertz and have AAA, car seats are free. We almost always use this option to avoid the car seat lugging hassle.

For the four year old, you could also consider the Bubble Bum inflatable booster (assuming that your four year old has reached 40 lbs). While I still use a five point harness for my almost five year old at home, the Bubble Bum comes in handy for travel.