We go to France alot, we never rent car seats, we take them with us. 2 kids. We got a car seat bag (in America I'm afraid) and we put it through as oversized luggage. Initially we put them in 2 Killeen Heavy Duty (Green) bags wrapped with sellotape and tied with a luggage strap. Quite cumbersome but at least we know how to fit out own carseats. Not such an issue now for our 5 year old as the car seat relies on the seat belt only rather than an integral harness, so renting might be 'safe' now, but depending on the sizes of your 3 and 4 year olds, you may be relying on an integral harness carseat, and if so I'd take your own. When we first travelled with kids, we rented them, and the car company cannot install them for insurance reasons, and 9 times out of 10 they don't have the manual, and then the way they install seem to be different to ours here, they have extra belt clips...
I think the rental price is generally 35-50 per rental.