Thanks for sharing about your dc here, and you are right to ponder placement - it worth the time. We are doing 3 guides easily and happily right now. I think when just starting HOD it's nice to begin with 2 guides if possible, but then LHTH is so short (but fun!) I almost don't count it in the mix.
There are a few options I can see...
Option 1: Place your 6 and 8 yo together in Beyond, and challenge your older ds by doing the right side of Bigger Hearts for his LA and math. Yes, this would be an easier year for your 8 yo, but the following year in Bigger Hearts, you can always have him do the extensions if you want, and it would allow you to do just 2 programs, with your 4 yo doing LHFHG when done with LHTH.
Option 2: Have your 8 yo do BHFHG, and place your 4 and 6 yo in LHTH, but have your 6 yo continue to do the Emerging Reader's Set and the right side of LHFHG. Yes, it would be very easy for your 6 yo, probably too easy, but the following year the 2 could be together in LHFHG with your older of the 2 doing DIHTOR 2/3 and the right side of Beyond to up the difficulty some. BHFHG will be your biggest teaching time, and it is an important guide to teach, IMO. Teaching BHFHG well means years of solid academic skills and successful independent work to follow in the subsequent HOD guides. Very worth it.

So, if you like this option, I'd more than likely place your oldest there and get going with that right away, which I know you already are thinking to do from what you've shared so far.
Option 3: Have your 8 yo do BHFHG. Have your middle ds start Beyond half-speed. You can do the left side of the Beyond plans one day, and the right side the next day. This will be easy to teach and will let your ds grow and mature into the guide. Your 4 yo can do LHTH and when finished with that begin LHFHG half-speed. I've taught all of the guides we've used half-speed at one point or another, sometimes for just a few weeks to get our feet wet, and sometimes we've taken 2 years to do a guide and done it half-speed the whole time. It is easy to do and very flexible. You can go full-speed anytime you want. This option would give you a year between reteaching a guide (not necessary but nice for us moms as teachers), so you'd eventually have one child in LHFHG, one in Bigger Hearts, and one in CTC down the road. This is nice because as your youngers need you more, your older is more independent because the older HOD guides plan for that.
These are a few ideas that came to mind!

You could always start with Option 2 and change to Option 3 once you get going if you think it would be better to do that and you feel ready. I hope something here helps! Placement is the only semi-tough thing about HOD, once you're started, the rest is cake.
In Christ,
Julie