Car hire in Japan - child /booster seats

Hi
My son is 4 and we'll be hiring a car in Hokkaido.
All car rental companies say on their websites that children to 4yo ( some say 3), to 100cm and 15kg should go in a child cat seat (ie forward facing 5 point harness seats) whereas those above 4yo, taller than 100cm/ over 15kg should be in booster seats.
My questions are:
1) are Japanese car seats smaller built than Australian car seats? My son is 102cm and 17kg but is still around 3cm before he reach the highest shoulder markers on a 0-4 carseat ( which will mean he needs to swap to a booster seat). I tried him on a booster seat in Australia and he's still under the minimum shoulder markers for a booster.

2) the picture on car hire websites for boosters are all backless and with no neck support. Is that across the board for all hire companies or do some places do booster seats that have a back/neck support?

I am very hesitant to hire a booster seat as i don't think he's ready but I'm also worried that if Japanese 0-4 seats are smaller then he won't fit in them?

Thanks

I recently did a lot of research into car seats for my infant daughter, and feel that Japan is MUCH more lenient about car seats compared with other countries. IMO, children should go in car seats until much older (up to as old as 12), and not switch to a booster until they can comfortably handle the seat belt. I also live out in the country, and am still continually shocked and dismayed by how many parents either flat out don't use any sort of car seat at all, or if they even bother, are using it wrong (front facing in the front seat for example). I even have talked directly to police at the driving center where I live, and they basically said they don't ever even bother to check or to fine, even though technically kids are supposed to be in them.

So... yeah. What I would do is email the car rental place directly and indicate that you'd like BOTH a car seat and a booster seat to be available to TRY, and that you'll rent the one which fits your son best. I'd try the car seat first, obviously, as if its forward facing and he's right on the cusp, he'll probably still fit. In my experience booster seats never have backs; their purpose is simply to lift the kid high enough that they can use the regular seat belt comfortably.

As an aside, recently I've noticed a few British-made car seats have become more available here, which follow international standards of safety. There are even two that are suitable for use until age 12, but every single Japanese brand I checked only went until 4 years of age. It might also be good to see if you can find out which brand of car seat they're offering, and do a little research online about it. It might be that they've got the seats configured a certain way, but they can be adjusted to a larger size. Probably not, but it doesn't hurt to check!

Thanks scarreddragon.

I'll definitely email the car hire companies to try both and hopefully i can fit him in the carseat.

Australian booster seats all have a back which would provide back/neck support until around 8 depending on height. We also have harnessed child seats that can keep them in 5 point harness till 8 and i have never seen the backless boosters that just lift the child up higher and as such i really don't want him in one of those boosters.

The other option is to bring my own from Australia but that's one extra thing to carry...
Anyway Thankyou.

I would see if they can tell you the brand/type of seat as well. Although increasingly rare, some car seats (usually older seats from Japanese brands) don't conform to ECE R44. I would assume that the rental companies (especially international ones) would have seats available that do, but I ran into a few when I was researching that are still being sold new in stores. To be honest, if the car rental is covering most of your trip, you might consider just bringing your own for safety and peace of mind. At the same time, I kinda feel like I'm probably being more pessimistic about it than warranted, and more than likely you'll be ok with the car seat at the rental place, so I don't think you should worry about it too much. But I'd still check and see if you can get more info from the rental companies you're considering!

Don't get me started on car seats in Japan...
We live in Australia, but stayed in Japan for 2 years, and sooooo many parents let kids NOT wear seatbelts or in child seat!! Kids were walking around, standing on seats, moving from 2nd row to 3rd row etc!!
And if babies/toddlers are asleep at home, many parents leave them at home and go out for a quick errands.

Anyway, when we rented a car with our kids, we used child seat, not booster seat.
Whatever the measurement and weight they give are just a guideline, and it comes down to how you as a parent feel comfortable. I'd go for child seat.