The ladies have done a great job of answering you!

One thing I'll just pop-in to say is that we rarely recommend that you do 3 of the lower level guides in a row (all at full-speed) as they take more teacher time than the upper guides. So, while it isn't difficult to run multiple guides, it does matter which particular guides those are!
I'm not saying that it wouldn't be possible to do those 3 guides at one time, but the following year it would get quite a bit more time intensive when you head into Beyond, Bigger, and Preparing. With that in mind, and looking down the road for the long haul, I see a couple of options for you to ponder before you decide to go the LHFHG, Beyond, Bigger route for the upcoming year.
One option would be to have your almost K student do LHTH and add in the 3R's from LHFHG to make the K year. This would give you the great Bible focus and skill-training, but would make sure that the 3R's are on right on level. You could add a phonics program, a K handwriting workbook, the Do It Carefully/Finding the Answers, and even the K Earlybird math with daily plans from the LHFHG guide to raise the level of LHTH. Then, the following year your child would head into LHFHG and do it with first grade options.

This plan would allow you more time to work with your olders and would keep you from having 3 guides in a row and may actually fit your younger better.
Then, you could more easily keep the Beyond and Bigger for your older two if desired.
Another option would be to place your older two together in Bigger Hearts, but do a 4 day plan. In this way, you would spill over 9 weeks into the next school year, which would allow your younger student time to mature a bit before heading into Preparing, but would also allow you to keep your older two together as a pair. This is also a good option, and it could be used in combination with the LHTH idea I shared above or separate from it.
Anyway, just a few thoughts as I'm looking down the road for you, in the hopes that you will have a good fit yet a manageable path for the future!
Blessings,
Carrie