This is the type of seat I have for my 5 year old - it would be fine for your 4 year olds unless you still want a 5 point harness for them. You can get them with isofix which is nice and that is all you will need until they outgrow car seats or if you prefer a booster once they are big enough.
I use a US car seat here in Switzerland. Our seat has LATCH connectors and are compatible with the local ISO Fix system. No one has ever checked he stickers on our car seats. One big advantage some US car seats have compared to those sold here im Switzerland is that they allow you to keep your child rear facing for a much longer period (up to 40 lbs). This also is the norm in Sweden. Studies show that this can greatly reduce the risk of spinal injury in case of an accident.
Our children "graduate" around 2-3 yrs old from a baby seat to the normal harness ones until they are big enough not to need one. So your 4 year old twins should be able to fit in a seat like DrMom's on arrival in CH until old enough to not need one.
For cheap seats go to www.Ricardo.ch . They don't sell well second hand and you can pick up some good ones for pennies compared to shop prices.
wow! really helpful info! Good to see that car seats are not too expensive in Switzerland - frankly, I was expecting much worse.
I will check my twins weight in kg to see what type of seat they would be in.
My Britax seats do not have the sticker, so I called Britax today to see if they could send me one. They said they sell different models of seats in the U.S. than in Europe, and different from Canada too. All countries have different requirements, and they make different seats to meet those specific requirements.
thank you everyone for the input, you have been very helpful!
I recently had a conversation with someone from TCS who explained that in fact, the seats are all the same, it's just that TCS doesn't have the time or resource to invest in testing new car seats - hence the limited selection available in Switzerland. The upside is that you can use European seats, and as other have said, who is really going to look at the stickers on your seats - even in the worst case scenario where you have an accident (and it's not like you're in a country where it would make sense to sue the car seat manufacturer in the event of an accident)