Well I don’t know antying about RS4K, but we use and love Apologia. We have used all three Animal books (Swimming, Flying and Land Animals) as well as Botany. We will be starting with Astronomy this year. We plan to use Anatomy next year, though the Chemistry book looks to be great too. Might try to fit some of both in actually.
I would say for a 6 and 4YO you really don’t need a science curriculum. I have heard wonderful things about the 106 days and if we had just those ages, I think I’d be trying that one for sure. But we have 6 children and do science together for all but our oldest, who is using Apoligia’s Gen. Science, which we don’t love but it is good and is what he needs.
So, my vote is to wait on Apologia and maybe consider it again. Please don’t discount it though altogether because many people including me and our children love it. You might want to take a look at it sometime from a friend who has it or at a store, but I would say your children are too young to fully appreciate it. I think the Apologia elementary books are written with the 3rd-6th grader in mind. I wanted to add as well that for us, Apologia is a starting point. We try to do many of the experiments, which greatly add to it, and also to further explore topics that we first read about in Apologia.
Just tonight, we revisited the seahorse in talking about its unusual reproductive means (don’t ask how this came up LOL! at dinner no less), and one of the books we consulted was our Swimming Creatures book. The real-life photographs also in the Apologia books are a great asset in my mind, but this is all just my two cents of course for whatever its worth!
Oh and don’t leave out the nature journals. I am so glad I got one for our two 6YOs last year. They will always be cherished by me (and them I hope). Such precious keepsakes to see nature through their eyes at 6 years old!