Best car seat for a 4 year old

[RewildingAmbridge]

DS is 3.5 and above average height, weight is average so should get to 4 with his current ERF seat. Question is which seat next?
I'm happy that we've had him rear facing to 4 but would consider FF for future. So many of the seats look flimsy but it might just be my imagination.
We have isofix, budget isn't an issue within reason. I also don't want one that's huge and very heavy as each grandma has him once a fortnight so the car seat is moved each week. Not worth buying two additional seats for each set of grandparents for the sake of one day per fortnight and they wouldn't want car seats in their cars permanently anyway.
Any recommendations appreciated!

Did you want him in a 25kg harnessed seat, or do you think he'll be ready for a HBB by 4? The Joie Bold is a good option for a 25kg FF seat and will convert to a booster afterwards. It is bulky though and as a harnessed seat would need installation with isofix, seatbelt and tether (). If you are wanting to move to a HBB but are a bit unsure, then some of the Britax and the Axkid seats have a belt positioner to keep the belt in the right place over the hips.

High backed boosters are pretty light and easy to move around compared to the harnessed type seats, so I wouldn't get a combination one if portability is a concern.

The best rated are Cybex, Britax, Joie and Besafe. Avoid the cheap Disney ones, Graco and the own brand models except the Halfords ones made by Joie are OK. I'd take him to a shop when he's close to outgrowing his seat and try some out. It's important that it's comfortable for him and the belt fits him well.

We have Cybex seats. They are well-rated and seem pretty sturdy.

I've had a look at cybex seats and they seem good, we're looking at the cybex solution z i-fix plus, my only concern is the lack of harness but this seems standard for group 2+. It just feels like a big jump from rear facing with harness to forward facing and just the usual seatbelt, am I just being PFB? 😬