I've been reading up on the SWM-8, and I'm not sure I understand everything that's involved in the set up.
Keep asking and reading, we can help you

They really are pretty simple to setup.
First of all, I believe it will work with 3 of our 4 boxes, which are d-12's. However, one of the boxes is a D-11, which doesn't work with the SWM-8?
Correct, the D12s should work fine, the D11 will not work with the SWM.
Second, I assume I'll have to run the SWM-8 outside, and then where the line comes into my wiring panel, I run the power inserter, and then run a splitter off of that to the 4 receivers. Is this correct?
Yes, if you got the SWM8, you would run it outside (it is weatherproof). The 4 lines would go from the dish to the SWM8, then the one output of the SWM8 would go via your one line into the house. Power Inserter would be put on this line in the house, and then you would split it like an antenna cable (just make sure you use proper approved splitters) out to your receivers.
If you went with the SWMLine LNB, you would just replace the LNB on the dish (simple...just 2 screws) and then run one line from the dish to the inside of your house, where again you would have the power inserter and use the same splitters out to the recievers. The SWMLine eliminates the need for the 4 runs from the Dish to the SWM8 module and eliminates the need for a separate SWM module at all.
Third, as kiljoy mentioned, the single run of cable may not be enough to handle the HD bandwidth, so I guess the SWM-8 won't provide a solution to that.
No, it should be just fine for HD. The SWM modules are made to work with existing wiring on the output side. You would just want to use good RG6 from the dish to the SWM-8 module if you go with the SWM-8.
I guess I should have had them install the 4 lines initially. However, the guy didn't exactly exude and air of competence. I didn't trust him to properly put that hole in my wall. That, and he wanted to put the dish on the front of my house.
There are circumstances where more lines can be a big problem, that is why there is now the SWM options