Don't agree that ONLY Top 4 would signify decent progress;if,let's say,Caroline finished the year #5 and was reasonably solid in every upcoming Slam,then I wouldn't call that 'stagnation'.However,if she merely holds her current ranking and never goes more than 1 or 2 spots above that,then yes,this year probably won't have amounted to much by season's end.....
How much (if at all) Caroline goes up in the rankings won't depend on Caroline's level of play alone though. Top three - forget about it. Serena, Masha and Vika have clearly seperated themselves from the rest of the field.
But top five also sounds pretty unlikely to me at this point. Radwanska is sorta in the position that Caroline was in a few years ago - rarely beating other members of the top five, but beating everyone else almost everytime. And Li Na has a newfound focus since she started working with Rodriguez and is now a lot more consistent than she used to be and a legitimate contender at every event she enters. Since all of those five are thousands of points in front of Caro our girl would not just have to play as well as them to catch up before the year is over, she would have to play better than them, or one of them anyway.
It's not impossible that Caro will overtake a few girls more directly in front of her, but even that is far from certain. Kerber might be a likely victim since she seems to be going through a slump herself. Kvitova - perhaps. But you know better than most that she's a peaks and valleys player, and if she's at a peak at the right time of the season it's back to glory glory hallelujah in no time as far as Petra is concerned. Many predicted that Errani would take a nosedive this year, but that hasn't really happened. On the contrary, and we're getting to what should be her best part of the season.
On the positive side for Caroline, she doesn't really have that much to defend for the next five or six months. Especially her record at the three remaining slams last year was dreadful. So it shouldn't be that hard to improve on her results there and make up some ground. But I don't expect, let alone count on her leapfrogging that many players in front of her in the rankings. If everything goes well she might overtake one or two of them and qualify for the YEC though.