Joie - Every Stage

Honestly a Joie stages/every stage/steadi is your best bet. They will all read face to around 4, longer if your child is low percentiles. They are all reasonably lightweight (<10kg) so can be carried through the airport (not exactly easily but still). We have the stages and it is seatbelt fitted, so no messing with isofix or tethers in a hire car.  None of them will fit to a stroller, that isn't available for any seat outside of group 0+ (i.e. baby bucket seats). Why do you need it to attach to a stroller anyway? If you are driving around surely you will hire a car and can leave it in there during the day? If you are taking taxis everywhere you may be able to check it into a luggage store at the waterpark, zoo etc.  I would not want to be carrying a seat around all day, by the time you get a seat that fits a 4yo it has to be massive to be able to protect them. So I'm afraid I don't think what you want exists

We’ve got a Britax Romer Max Safe Pro which is what is used 90% of the time because its in my car, its a really good seat, scores high on safety tests, easy to fit and will last at least 4 years which I think I saw you say you want. They do sales often too so you can often get it for around £180. We also have a Joie Every Stage for my husbands car which works fine, but I do prefer the Britax