I'm still learning the ins and outs of this, but as your set up is somewhat similar to mine I'll give you my thoughts. First, assuming the SD cable boxes get more channels than just a TV tuner you probably want to go with having them at the MD, with s-video out from the cable box to an s-video capture card (PVR-150 or something) in the MD.
For the HD receivers you have a couple options. First, you can do the same thing as the SD boxes, but all the recordings are going to be in SD. You can still watch live TV in hi-def, via input switching. In theory you can control it via MD and LinuxMCE will swap to the hi-def input while watching live TV, and switch back to the s-video for overlaying guide info, controlling lights, or any other lmce function. Unfortunately this didn't work out of the box for me and I haven't spent any time getting it to work. For now I just hit the input button and watch it live.
Another better solution would be to use the Hauppauge HD PVR to capture component video the same way you can capture an s-video stream. Component video is limited to 720p or 1080i (component spec, not a HD-PVR introduced limitation). At $250 or so for the box, plus huge disk requirements it's far more expensive than s-video capture, and still isn't true 1080p, but it's the best quality you can record all channels as far as I know.
For yet another option, you can go with an HD TV tuner, HD Homerun, or a Hauppauge card or whatever. In my experience and from what I've read, you will be limited to channels 2-99 because Comcast treats those as local sends encrypted QAM for channels 100 and above. There are some HD channels in the sub 100 range (they show up as things like 7-3 on my HD TV), but you'd have to map them somehow to the "real" Comcast channel numbers in Schedules Direct, and I've heard they can move around, go away, new ones appear, etc.
I'm using a laptop for one of my MDs, so I'll be configuring it to use the Firewire port on the STB at that TV. In my testing with Firewire on my core/md hybrid, Firewire is not 100% reliable, is limited to the same 2-99 channels, cannot be configured at the same time as s-video (or component capture) as Myth can't be told both inputs use the same STB, but has better quality than s-video capture.
If you want access to all your channels and don't mind having the extra STB at those 4 TVs, I'd suggest you just add an s-video or component capture card then control the STB via USB-UIRT, serial or Firewire. I haven't tried just controlling the STB via Firewire without capturing it, but I hear it's possible. I have tried using Firewire to capture channels above 99, but instead of just not working it seems to hang Myth. It is possible to control some STBs via serial connection, but I haven't tried that yet.
At this point if everything is configured correct and you do all your TV watching through Myth, you'll have access to anything recorded anywhere. If you are watching a channel on one STB, I believe you can go into any other MD and watch the recording. It's not going to broadcast to all tvs, but by watching the recording and fast forwarding to the end it's basically the same result.
Disclaimer: I'm still learning all this, so I may have some things wrong and there may be other options. Someone please correct me where needed.