Traveling with a 4 year old in Europe

Do taxis where you live have car seats?

Probably best to plan on bringing your own.

Get a light weight car seat and carry it with you. You will definitely need it for the car, I wouldn't trust one from a rental car company. I only remember one taxi having a booster seat for my daughter a year or two ago when she was 4 or 5. I think most taxi's are exempt from the car seat requirement, at least in a lot of US cities they are. You won't need it for the train but of course you can use it for the airplane. Use an umbrella stroller for walking around. Don't forget to get one of the umbrella covers to keep them dry if it rains. It was well worth the money for us when we were using a stroller for our daughter.

Have fun!

There's also a wheeled contraption that you put yr regular car seat on so that you can pull child in car seat thru the airport

Just don’t use taxis and rent a car seat with your rental car. For longer car rental periods, we have always just bought a car seat and left it behind.

Make sure stroller had large wide wheels to handle cobblestones or the small pea gravel found in gardens. I have seen countless parents get stuck or have to fight to get their stroller through it. We used one like a jogging stroller.

Modes of transport will include airplanes, trains, and cars. Rick’s
travel books mention young children must use car seats in cars while
in Europe

Rules differ per country, even within Europe or the EU. For the Netherlands: children under 1.35 m (4 ft 5) have to be in a child's seat. There are however exceptions on this rule. One of them is that children under 3 yo do not need to be in a children's seat when travelling in a taxi.