For your 4th grader, I think if he is a very good reader and loves to read chapter books, I don't think the level 4/5 would be out of line. With Preparing though - he will be reading quite a bit to himself with history and science though through the year. My oldest did Preparing last year and she read the level 3 books. Some were longer and harder, some where not. She was not a voracious reader

But she loved everything she read

As for grammar, I think R&S 3 would be fine. My oldest didn't have exposure to formal grammar until last year and it went well. The R&S book does a very nice job of hand holding.
As for your other two kids, I see that you mentioned you would put the 6th grader in a higher level and then bump down to CTC after that. I personally would not do that. Each level increases with difficulty. So if you bump them down, it may be shocking on them to complete the higher level after that. I would be afraid you would loose steam. If you really want to keep your older two together, I would put them both in Resurrection to Reformation now and give the older child the extensions. With MTMM, it can easily be modified to make it high school worthy and many moms have done this. You would just want to be sure to add in economics during this year, use DITHOR but update the books to be high school worthy books, you could modify science or skip it and use the science from WG, and then add in Spanish from WG. Modifying may sound daunting but this would really be the only year with significant changes. However, some people have had kids stay together and find that after RevtoRev, they may skip MTMM because a particular child is ready for High School. You could do that too instead if your oldest was ready for that workload. But many others prefer to just modify MTMM. So I think there is a lot of freedom.