Welcome to the HOD Board!

I'm glad you are enjoying Beyond with your dc. You are so wise to consider this now, as this is the perfect time to set up a long-term plan for combining down the road. If you would like to combine your 4.5 yo with your 7 yo long-term, I'd start to slow down Beyond to half-speed. Many moms do this and enjoy it because it makes combining feasible for both dc. I know that doing full-speed Beyond is working now, but when your 2 dc get to Bigger Hearts, your dd's lack of writing maturity (which is 100% normal for her young age) will catch up with her, and it will make doing Bigger Hearts on up difficult. Writing becomes more of a key component in Bigger Hearts (and on up), and Charlotte Mason skills are tied to it, so you don't want little pumpkin being stretched way too far and missing those introductory teaching lessons needed for those skills.
My middle ds was reading long chapter books by the time he was 4.5 yo, and when he got to Bigger Hearts at 6 1/2 yo he was writing quite well, but I still slowed it down to half-speed for him because it would have been way too much otherwise. I think the youngest age suggested to do Bigger Hearts (7 yo) is very accurate - a child doing it younger than that would more than likely be headed for overload.

So, I tell you all that just so the advice I'm about to give makes some sense.

I would start doing a portion of Beyond half-speed with your dc right away. The easiest way to do this would be to alternate the left and right side of the plans, doing the left side of the plans one day, and the right side of the plans the next day. You could continue to do full-speed math, reading, LA, with your ds, but slow down to half-speed with your dd. So, he would probably complete Spelling List 1 this year and his 1 x a week grammar lesson (on Day 5 always). Then the next year, he could do R & S English 2 and Spelling List 2.
You could decide what you want to do with math - either full or half-speed. I wouldn't rush to the upper levels of Singapore myself, as it gets very challenging - but you will know what you want to do there. For Storytime, you could go full-speed, and then next year when you are finishing out the rest of Beyond, you could just choose a different Storytime set to read aloud. I'd also work toward getting dd to write, as that will be the biggest hurdle to getting to Bigger Hearts too early. You could always slow to half-speed for Bigger Hearts too, but I don't see any way that a 5 1/2 yo could get what is intended to be learned from Bigger Hearts, even if she is incredibly advanced, I just think she'd miss so much due to maturity and lack of writing skills, so it seems to make the best sense to slow Beyond down. Here's a post that explains the difference in writing between Beyond and Bigger that may be explain this better too:
Amount of writing in BHFHG:
viewtopic.php?f=6&t=3005
viewtopic.php?f=6&t=1516&p=11829
As far as your younger 2, yes, I do think that makes good sense to combine them as they are quite close in age.
These are just some ideas for you to ponder, but keep asking questions here until you come to a peace about a placement plan for your sweeties. HOD has blessed our home so much, and I pray the same for yours!

I look forward to getting to know you here - I think you'll find this board to be full of encouragers.
In Christ,
Julie