Why can't you install a GFCI in a box that has more than 4 wires (not including ground)?
Instructions are obtuse because of UL

This has to do with the device's UL listing.  UL only approves devices for certain applications; i.e. you can't use an Alumiconn to splice 9600V wiring. Two things assure you stay within those applications:  First NEC 110.3(B) which requires you obey the labeling and instructions, and second, UL also approves the labeling and instructions as part of the listing.  That is because, generally, they only test for conditions described in the instructions (e.g. they don't test Alumiconn for 9600V splices).  Anyway, if you wonder why some instructions are so stilted and weird, that is why. 

When a manufacturer writes instructions likely to be read by amateurs, UL gives it extra scrutiny.  It is UL who is imposing the "See electrician for >4 wires" issue.  You have to hook up the two LINE terminals or the device won't function.  With only 2 additional wires, UL feels comfortable with the instructions.  >4, there become several wrong/dangerous ways to hook it up. 

Just use LINE.  Leave LOAD alone.

Anyway, for you, it is simplicity itself.  Everything goes onto LINE.  Don't even remove the warning tape from the LOAD terminals.  So you would just pigtail. 

Do not use the LOAD terminals at all unless you have a specific reason to do so, and only for the pairs of wires where that reason exists.  The instructions will say something else; they want you to ignorantly protect parts of your circuit without realizing you are doing so, and because of the side-effects, that's something I do not agree with. 

Why not use LOAD for everything? Well, you're perfectly allowed to; and you're advised to by the instructions.  That will place everything downline into the GFCI protected zone.  That seems oh, so clever: Free protection! And that is the purpose of having LOAD terminals!  However, this has a dark side, if those downline loads don't need GFCI protection.  Any trivial leakage from those things will stack i.e. cumulate, increasing your risk of nuisance trips.  Nuisance trips drive people crazy.  Do people do a laborious bug hunt on each leg of the downline that wasn't even part of the project in the first place, to suss out every last dirty contact or NM wire that's getting wet, or stick isolation transformers on their refrigerators?  No, they don't; they rip out the GFCI. 

So "GFCI everything" is a grand theory, but not something you can really live with.  UL doesn't care about that. They say "you should do the bug hunt".  UL can come and do the bug hunt, IMO.  

What is a good idea to not protect? 


Grounded hardwired loads, esp. lighting where dangerous power tools operate (e.g. curling irons)
Any downline outlet which has its own GFCI protection
Refrigerators that are cord-and-plug connected (post NEC 2014 you'll need to get a variance if the fridge is in a location where all receps need GFCI)
Safety equipment of any kind (including refrigerators).  It won't do to have an emergency fire pump trip a GFCI the moment it starts; the purpose of the gadget is to protect from fire, not electrical shocks.