Ken's Comments ...
H&M-S show three situations, each relating to 4 v. 3.
The plus point comes like in diagram No. 11 and 12. In #11, note that if Black moves to e5, White can take, and Black cannot retake. This hinders the pawn from advancing. This becomes a countable point. Maybe think of it like this: If White has more firepower on (here) e5, than Black (the disputed square), so the pawn can't move, it is a countable point. (Like COA Center Domination of the one square, e5).
In #12, if Black moves the d6 pawn a second time to d5, he is down a tempo, and thus will be at a disadvantage. You would not count the tempo advantage for White at this stage, because it hasn't been realized, but you know if Black moves, he will be behind in development by one tempo, so you count that as the point now (4 v. 3). When he makes the move d5, you no longer have the 4 v. 3 point, but you have gained the tempo point. So one point converts to the other. Tempo refers to how many times each pawn or piece moves, especially in the opening. It is an advantage because it hinders the side on the 3rd rank.
No. 13 is totally different than No.'s 11 & 12. Black has not moved his center pawn (on d7). Even though White has a pawn on e5, Black only needs one move to go from d7 to d5, instead of two. Also, when he gets to d5, as long as the Knight is on d4, he is attacked once by the pawn on e4, and is defended once by his Queen (COA Center Domination on d5 would be equal). Thus there would be no point for White.
To sum up, 4 v. 3 means the opposing pawn must be on an adjacent file, and must be on the 3rd rank (not second), and the single square to the diagonal of the pawn on 4th, ahead of the pawn on 3rd, must be COA Center Dominated. If all three of these criteria are met, count yourself a point for 4 v. 3. If you reread p. 25 and 26 with this in mind, I believe it will make more sense.