Yes that does make sense. It's not very clear, is it?
Any seat which can be used until 4 years won't be the kind that goes onto a pushchair to make a travel system, though. Even i-size seats which work with a travel system are just the infant carrier kind with handle. These are designed to go up to about a year, 18 months if you're lucky (but they get pretty unwieldy before then anyway).
If you want a birth-4 years seat then you can look at either i-size or R44 (group system) seats, but either way, these are designed to stay in the car and don't go onto a pushchair, the seats are too big and heavy, and wouldn't stand the bashing around that a travel system seat gets in everyday use. They can be a practical choice but in your situation with wanting to switch between cars, I'd recommend going for an infant carrier seat (either i-size or Group 0+, there is really nothing between them at the moment) and then when your baby is around a year old look again at what's on the market in terms of second stage seats. It might be that in a year or more from now there is something much better out there anyway.
At the moment there's not really much difference between i-size and R44 approved seats. i-size requires some extra testing, but most of the big manufacturers already use these extra tests anyway. The main plus of i-size going forwards is that R44 seats are tied to the "group" system which has set weight categories (0-13kg, 9-18kg, 15-25kg, 22-36kg) which can make it hard to find non-standard seats for bigger children or inbetween categories. For example, when my DS was younger it was difficult to find a seat which would allow a child to stay rear facing past about a year. Now it's more common but it took a long time because most people didn't want a seat which was rear facing right to 4 years old, and there was no middle ground possible. The current issue is that most seats on the market stop using a 5-point harness at the 18kg/100cm mark which can be too small for some 3 year olds or even some big 2 year olds, but there is so little demand for seats which harness to 25kg/125cm (the next possibility) that manufacturers are not motivated to produce them. The idea with i-size is that as demand grows, different manufacturers will compete to produce seats which use the higher safety standards for longer, perhaps starting at 105cm, 110cm, 115cm, etc. But because R44 is still dominant, there is not much differentation yet. We're seeing a little - some i-size seats designed for toddlers go up to 18.5kg for example, but IMO there's nothing which is definitely worth the bump in price at the moment. I'd get the baby seat, and wait and see what you can get in toddler seats later.
Hope that helps - feel free to ask if you have any more questions.