I think we got into this exact discussion the other day...
This isn't an MMO you shouldn't expect the same design principles and the multiplayer will clearly have different priorities when it comes to the combat.
Also you seem to be assuming when they were play testing the single player combat in the multiplayer builds that they did so with 4 abilities... you really seem to have a massively low opinion of these developers.
As for your difficulty remarks... have you played it on the hardest difficulty, do you know how difficult the game will be? No. The game isn't out.
The problem with this philosophy is that I'm not the one claiming they balanced combat in MP, they are. 4 abilities is the limit, in MP, with the other four slots being for potions, at least, that's what I read concerning MP, do you have something different? Are you going to be in a MP match changing the skills on my character? What tactics are you going to have to change for the group synergy in MP? Will changing tactics on your screen suddenly make my character do something I didn't expect? No? So none of these mechanics even come into question during MP, and none of them can be tested to be sure that their "it works great in MP" ideas carry over to the SP campaign, such as "Hey, the radial menu is too clunky for MP, we're going to have to cut it", "yeah, but when we do that, the console gamers are going to have a fit because the PC has access to more skills", "Oh, we can fix that, we'll just limit it to 8, nobody will notice that", "Heh, it's the BSN, we'd better not release that information until we set the hook with MP".
How do you even pretend to build a SP game in MP mode? It made the DAMP people even DAMPer, but really, it didn't bring anything to the table for the SP game, it took things away. Well, technically, I suppose it didn't, it just made it so that those things would never be available, like truly customizable builds, as opposed to "customized for this particular part of the game" builds. DAMP wasn't even off the presses yet and people were like "Can we solo it"... Do you honestly believe people won't be soloing the SP too? So all these arbitrary design decisions made in MP testing don't do anything but limit those of us that just play the game, or, more to the point, I guess, force us to remove focus on specific builds in favor of more jack of all trades builds, unless we want to just press 2 or 3 buttons, or ask for a macro system to do it for us, when our flavor build doesn't work, and if you're wondering, Specialist Mages that stay true to their specialization the whole game? They're flavor builds now.
Oh, and as an aside; if they're building the game to force more party synergy, then I should totally expect it to have more MMO like qualities, because MMOs are all about party play, or they're supposed to be.