Hi Lisa! You are getting such good help here, aren't these ladies amazing? I wanted to encourage you that we do have many moms with larger families doing multiple HOD guides and enjoying their lives. I am doing 3 guides, but I only have 3 sons, so I realize that can be different. All I can say is HOD makes doing multiple guides easy on us moms. The younger guides do not take as long to do, and they really put our teaching time into training dc to incrementally learn skills so independence is a successful endeavor in the older guides. The older guides have an excellent balance of independent, semi-independent, and teacher-directed work within each day, as you can see by the labeled boxes (I, S, T).
Our dc love school, and they are each thriving as they are truly able to learn so much and do all that is asked of each of them within their guides. There is no tweaking. No prep time. No planning time. No making dc listen to things they cannot understand, nor having them do subbed in skills because they can't do the activities. There's also no need to skip art projects, science experiments, and hands-on work (which BTW I had to do routinely with another program I used that was NOT HOD).

The reason these things don't get skipped is because you do not have to do them. They are written so the child can do them. They also don't use strange ingredients, materials, or require trips to the grocery store or the library. You can do them right with what you have on hand in your home, and many substitutions for materials are routinely suggested making it even easier to do them. They also are a balanced part of your day. You will never have an art project that takes 2 hours or 2 weeks, and then have one that takes 5 minutes. All of the boxes of the plans of the guide are kept in balance with one another, and CM's short lesson of teaching skills in a rotating manner is used. So, you can always plan each box taking "x" amount of time. For example, here is a rundown of the suggested time allotments for PHFHG:
PHFHG the time it takes...
viewtopic.php?f=6&t=4244&p=31219#p31219
And here is the time BLHFHG takes...
Beyond, time it takes:
viewtopic.php?f=11&t=162
viewtopic.php?f=6&t=4390&p=32417
Have you gotten a chance to look at the first week of plans of the guides you are considering? That is so helpful in choosing. When choosing between CTC and PHFHG, look carefully at the "I" boxes, and see if your oldest dd can 100% do those independently. Likewise, check out the "S" boxes, and see if you can give your dd an intro to the skills in those boxes, and if she can then finish them out on her own. For the "T" boxes, you can see if she will be ready for the teaching you will be doing there. I think you will be able to see whether she places in CTC or PHFHG then, but please keep asking questions until you know for sure!
http://www.heartofdakota.com/pdf/PHFHG-sample-week.pdf
http://www.heartofdakota.com/pdf/CTC-First-Week.pdf
As far as your next 2 sons, I think you'd enjoy doing Beyond with the 2 of them, but you can up the LA/math for your 8 yo by using the right side of the Bigger Hearts guide for those subjects, as well as its Appendix for the dication. This is very easy to do, and it makes for a good combining situation for both.
Eventually, you can do LHTH with the littles, but that can be started later. It is super easy to do, and I look at it as a great way to get in my "mommy time" with the littles in an easy way that teaches them solid beginning skills like letter recognition/formation, colors, shapes, math concepts, Biblical history with comprehension questions, rhymes that get them movin', art activities, Bible activities, fun music to sing along to, etc. - BUT all within like 20 minutes a day.
We had a fantastic year this year using 3 HOD guides! I think would find it very doable to begin with 2 HOD guides, and add LHTH whenever you wanted to. In case it would be helpful, I thought I'd link my schedule here, but just disregard it if it is not helpful...
HOD is such a great way to teach dc! It lets me keep my olders challenged, yet meet my younger dc's needs too. It balances my time among my dc so well, and all are getting what they need from me in a way that we all find enjoyable. The littles are not left behind or dragged along, but rather are getting the same great early learning my oldest received from me when he was little. I love our homeschooling with HOD, and I think you will too! Please keep asking questions until you have a good fit figured out - this board is full of wise, encouraging, Christian ladies that love to help.

Placement takes a bit with HOD, but then the whole year you'll be so glad you have everyone smooth sailing along! HTH!
In Christ,
Julie