4...Bf7! was better!

Bb4 is the threat winning a piece.

Bb4 is coming and d5 will suffer.

With the bishop trade, the weaknesses on the light squares on the queenside will be more evident, especially the c6-square, a fantastic outpost for a knight. This is a pattern that we have seen in previous examples as well.

Anticipating the check along the diagonal.

3...Qb7+ 4.Qf3 This is the idea. After the queen trade, the c6-square will again be accessible for the knight.

Guaranteeing an outpost on c6 for the knight. From there, it will block the c-file and there will be some Ne7+ ideas in the air.

A natural move to protect our pawn chain.

Botvinnik is not interested at all in the a-file. The plan is Nc4-Nd6, with a very active position.