Does the marriage contract become void, according to Imam Malik, due to delaying the announcement of the marriage and keeping it secret, knowing that there was an agreement to announce the marriage after four months?

Announcing the marriage (نكاح) is recommended according to the Malikis and others. If its announcement is delayed or does not happen at all, the marriage is not invalidated. No one has stated that announcing the marriage is obligatory; rather, the command in "Announce the marriage" is for recommendation. There is no opinion that the announcement must occur at the time of the contract; instead, its recommendation is general and includes the period after the contract.