This is quite possible, probable even given results so far this year. Murray's performances at the slams are not good enough to keep him in the top four. He defends a huge amount of points from now until the end of the year, most of them at Masters and ATP500/250 events.
A couple of bad performances and he could actually drop as low as 8 by the end of the year.
You are funny, boogers and sometimes, hilarious. I guess you forget when write this sort of stuff that you were just as much negative when Andy lost a match in a tournament where he didn't have any points to defend. The points are just a cover up and to validate the fact that you are typically a very negative person and you yourself have admitted not so long ago. But still, I will take your post at face value and answer the highlighted part above.
Let me tell you first though that you are one of the few guys (and they know very well who they are) who resort to worst case scenario first. And I've also seen other people worrying about things that may or may not happen like ever, but they worry nonetheless if there's even a remote possibility to it. I am not sure if they are just bunch of worry-warts or they just simply get so bored that they feel the need to cook something up, so that they can keep themselves busy thinking about some irrelevant matters.
And now going to the worst possible case scenario above, what's the worse that can happen if Andy indeed slides down to rank 6th or 7th or 8th or even out of top ten? You think it's really that vital that he stays at the top of ranking and it's the only way he'll ever win a Slam? Not quite. The two critical elements that are holding Andy really back have nothing to do with his ranking and they are: one, pressure and we all know by now how deadly that is and can be; two, no matter where he sits at the ranking bar and no matter how comfortable he is there, he will still have to face at least one of the top players en route to final if not two. There's simply no escaping from those two realities.
But to make your case even weaker, let me give a list of players who won their first Slam regardless where they were in terms rankings and almost all of them are legends:
Pete Sampras: ranked 12 (and we all know the tough line up he had to deal with to win his first Slam)
Andre Agassi: ranked 12
Boris Becker: ranked 20 (wasn't even seeded at Wimbledon)
Stefan Edberg: ranked 6
Jim Courier: ranked 9
Goran Ivanisevic: WC (and he beat Roddick, Rusedski, Safin, Henman and fianlly, Rafter to win his only Slam)
That is just to show you that if someone is meant to win a Slam or many, nothing can possibly stand in their way if they are meant and up for it. So if Andy is to win a Slam, he will win it regardless and in fact, the lower he's ranked the better it is for him, because no one will expect anything from him, since he has serious problems dealing with the pressure. We don't need another Wozniacki at the top and I really hope Andy understands the burden of being one of the top players (he really knows and that's why he doesn't mind taking some of the pressure off from time to time) and the responsibilities that come with it. He doesn't need it right now because he has yet to win his first Slam. Any idiot should understand this, really.