Moroni 10:4 is not so much of a promise as it is a manipulative device. It promises a particular result if certain terms are met. But the terms reflect on the seeker's integrity as regards both his sincerity and resolve, and on his faith in Christ. To be willing to rely on the promise of this verse as a test for the Book of Mormon's truthfulness one must already have concluded somehow that its instruction is valid and its promise reliable. That is, one must already believe in the "truthfulness" of this verse.
If the verse is true, then the only possible explanation for failing to obtain the result promised is a failure to meet the terms. That is, one must lack a sincere heart, and/or real intent, and/or faith in Christ. If one believes the verse is true then one must obtain the answer promised, or face an embarrassing judgement of one's sincerity, intent, or faith in Christ. The seeker is forced into convincing himself he has had some kind of manifestation from God, just to vindicate his own character. Or worse, he is moved to a frame of mind where he will gladly and indiscriminately embrace any supernatural manifestation as though it were from God.
Plain reason, not to mention all the force of Scripture's revelation of the character of God, testifies that God would not, does not, use such manipulative mind/ego games against the human family to bring them to believe the truth. God does not approve, and truth does not need, such machinations.
And will you do the same for the Book of ABRAHAM?