Only one of the four is "true", meaning that only one of them holds a different view.

In the question:

Grandma: It's better to learn piano; that is, grandma agrees with learning piano.

Grandma: If you learn sight-reading, then learn piano; i.e. Grandma is agreeing that learning piano and learning sight-reading is better than just learning sight-reading.

Grandma: You shouldn't learn piano, you should learn sight-reading; i.e. grandma is against learning piano, and supports learning sight-reading.

Grandpa: should not learn sight-reading; i.e. Grandpa is against learning sight-reading and agrees with learning piano by default.

There is only one "true" in the question, that is, there is one person who maintains a different opinion, and among the four people, only grandma holds a different opinion: learn to sing sight-reading, not to learn piano. （

The question is analyzed in terms of contradiction, which should be understood as one person being in conflict with all the others, rather than simply saying that grandma and grandma have a contradictory relationship.