The Red Sox have, at the least, a very good farm system. Keith Law ranked it the 10th-best in the game at present, even after a few years of promotions that have seen Xander Bogaerts, Mookie Betts, Eduardo Rodriguez, Blake Swihart, and Henry Owens lose their prospect status and in some cases, become established major-league players. This is also even after trading top-100 (and top-50, by some accounts) prospects like Javier Guerra and Manuel Margot for closer Craig Kimbrel.
Some outlets think Boston's farm is even better than "very good", though, even after all this. Baseball America is one such outlet, as they just ranked the Sox as the fourth-best farm system in the game.
You will not be surprised by who they cite as the reason for this lofty ranking. Yoan Moncada, Anderson Espinoza, and Rafael Devers, Boston's three major international signings from the last few signing periods, are all major prospects that any team would be happy to have. There is also Andrew Benintendi, who was drafted seventh overall in 2015's June draft after a breakout season at Arkansas, one he followed up with a massive pro debut that saw him finish the year in full-season ball.
There are a few other name drops in here as well -- not necessarily "top prospects" name drops, but still, they got mentions. Wendell Rijo and Mauricio Dubon were both named as reasons why second base is Boston's strongest position on the farm -- though, admittedly, that is mostly due to top prospect Yoan Moncada. Travis Lakins, drafted in the sixth round out of Ohio State last June, is their breakout candidate thanks to his "low-to-mid-90s fastball, potentially plus curve and athletic delivery."
Law's top-100 features prospects older than Betts, Bogaerts
The Red Sox "only" have four top-100 prospects on Law's list, but don't worry. They're still plenty young and plenty promising.
This seems a little high to me as far as team rankings go, but it's possible Baseball America loves Boston's five-through-10 prospects to the degree that they get some extra credit. That, or they think all four of Moncada, Devers, Espinoza, and Benintendi are already top-30 guys. We won't know the latter for a bit, not until BA releases their top-100, but we can guess as to who else they might love that is driving the Sox all the way to fourth.
Michael Kopech is the most obvious name, as he's considered a top-100 prospect by some already. Brian Johnson was on the top-100 list a year ago, and while he suffered an injury in 2015, his performance should not have given BA any reason to like him less. It's possible they like Sam Travis' odds -- he's still just 22, and was successful in his first taste of Double-A in his first full year in the pros.
They could also be driving the Luis Alexander Basabe bus -- remember, Baseball Prospectus' Chris Crawford saw Basabe as a potential top-100 prospect next year, so while BA is unlikely to have already gone that far, their appreciation of his promise might have, in conjunction with the rest, helped bump Boston up further in these rankings.
So, maybe the Sox have seven top-100 prospects again, even after losing Guerra and Margot and promoting Rodriguez, Owens, and Swihart in the same summer. That, plus some early Basabe love, is the best guess as to why they made it all the way to number four when Law, who certainly appreciates the system, had them 10th.
We'll know for sure when the top-100 is out, but educated guessing our way through things will certainly suffice on February 12.