And you are allowed to bring your seats with you and use them legally on the tourist's visa. It's when you get your resident visa that you'll need to follow the resident's laws. That took until the beginning of May for us, so four months. So if you're not certain, you can bring them, use them while you check out German and Swedish options, then make your decision before the law switches over for you.
This is incorrect and has been discussed many times before. This is what apply:
It's illegal to use a car seat in Europe which is not certified ECE R44. Like car seats with Canadian or US certification. It doesn't matter if you are in EU for a day, a month or a year. It doesn't matter if you are a tourist or a resident. It's irrelevant if you have a tourist visa or a resident visa. It's illegal regardless of these circumstances.
Those in the military or diplomats are the only ones who
might be exempt from the ECE R44 certification. There is no "switch" in rules between your status as a tourist or a resident. This is simply incorrect.
A four year old should be harnessed, and ideally with a top tether (if you get to pick your car, I'd look for one with tethers, they won't be automatic like they are in Canada). And of course kids should rear face for at least two years, and ideally longer. German seats won't allow for that.
The AAP recommendation of rear facing to age 2 or longer was Stone Age when it arrived. Not surprising since the goal of AAP seem to be to confuse parents as much as possible and be way, way way behind reality. Kids should rear face to age 4 or longer which we have known for a very long time.
A child at age four is not any safer in a harnessed seats compared to a high back booster. This is a US myth which is incorrect. There is no data, stats, research or real life experiences supporting it. A four year old child would be FAR safer rear facing but if forward facing a HBB is just as safe, or safer, than a harnessed seat.
The Swedes have focused intesely on car seat safety since 1960 and started keeping kids rear facing in 1965. The car seat safety is the envy of the world and so is the safety record. Fatalties for young children, up to age 8 or so, is basically zero each year.
Forward facing seats with harness for older children are not sold in the country for the simple reason of not being any safer. They are considered less safe by the experts over here largely to the extremely high neck loads. Other factors which make HBB a bettter choice are far lower prices, lower weight, easier installation and easy movement between vehicles.
There is no reason to use a harnessed seat FF seat for an older child unless the child has special needs. Doesn't matter what country is discussed.
The opinion of the car seat safety obsessed Swedes could of course be ignored. Lots of great advice is ignored. But it's likely not a good idea. The obsesson with keeping kids safe in cars and half century long intense focus on child safety has been very educational. The safety record speaks for itself.
The Swedish advice of keeping kids rear facing for a long time was laughed at for 30+ years. Now there is no laughing and instead parents are trying to follow these habits. If harnessed seats were safer for older kids we would not only use them in the country but we would also tell others to use them as well. Simply because it would save the lives of many children.
A Canadian rear facing seat will likely work fine although it's illegal. No one is likely to notice or care. Try to keep the little one rear facing for as long as possible. If you prefer EU seats there are many to select from. Both Swedish and others.
Use a harnessed seat for your older child if you prefer. Safety is not any better than a HBB but some parents still feel better about it. If your HBB is Canadian or from EU will not make a difference safety wise.