Yes I'd put the 4yo in the Advansafix - it's one of the better ones for 5 point harness use.
HBBs are more about the fit they offer than much inherent safety. Anything from Britax, Cybex, Joie, Maxi Cosi or Recaro is good - but ideally take child and relevant car to a shop and try them out to see what's comfy and what will fit with the seatbelt placement in your car. Nothing worse than spending out on something and then you find it has an awkward fit with the seatbelt and it annoys you for the next 6 years.
BTW, the 4yo can keep using the 2WayPearl as long as their eyes are lower than the top edge of the seat, and their shoulders aren't taller than the top setting for the shoulder straps - they are well within the limits of it height/weight wise, and probably will be for another year or more.
If you have the i-size advansafix (green belt guides), the 6yo is over the height limit for the harness though not over the weight limit yet, which is ~20kg depending on exact model. If you have the R44 version (red belt guides) then they are fine to continue to use it regardless of height with the same rules - eye line must be level at most, and shoulders must be level or lower than the shoulder straps at their highest setting - but max 18kg. It's likely you're close to this limit for 6yo, anyway.
I would actually say Advansafix is better than Joie Bold if the child is within the limits of it - it might be worth swapping the Advansafix into the main car, and the second Bold back to the other car, which would possibly mean you don't need to buy anything yet, because you could keep 4yo in the Pearl and 6yo in the Bold in car 2.