almost 4 yr old car seat question

[roundabout1]

Hi Forgive me if this is in the wrong place. DD2 is 4 next week and is still in her Britax rear/front facing car seat. We are now having to swap between cars regularly and dp has a bad back and the seat is very heavy. He has suggested that dd2 moves into dd1's hi backed booster that she no longer uses. We have tried this but I am not very happy about it as dd doesn't sit very well in it. I know dd1 moved into the same seat at almost the exact age but she has always been much chunkier and just holds herself better. DD2 has always been a bit floppy in a car seat as she just is very relaxed. So I have been looking at the 1/2/3 stage seat so a high back booster but with a harness but they all seem to say harness up to 15kg which is dd2's weight. Does anyone know if the harnesses generally fit for longer than that? To give an example dd2 in her Britax rear/front seat her straps aren't even on the highest setting yet. I hope this is clear, I am getting myself very confused over it all !

The harnesses are usually suitable until 18kg, but can be used with 3 point belt from 15kg so there's quite a big overlap where it can be used either way.

DS is 4.5 and is still in a harnessed seat, most are up to 18kg, with HBB being from 15kg minimum

it's done mainly on height and weight i think.

i have taken DS to mothercare in the past, they were very helpful and gave me lots of advice, measured DS etc. even when it became clear that i wasn't ready to buy anything the first time.

Can you afford a second seat? I'd rather that than put her in a seat she isn't ready for.

Thank you, sorry have had no internet. We now have a second seat - a high backed booster with a harness, thank you :)