Why did Iblis mention four directions in his challenge and not six, and why did he use "min" (from) with "amām wa khalf" (in front and behind) and "ʿan" (from/on) with "aymān wa shamāʾil" (right and left), and did Iblis have knowledge of the Unseen to know that most of the nation would not be grateful?

The meaning of "Then I will come to them from before them and from behind them and on their right and on their left" is to turn people away from the truth, to make them desire this worldly life, to make them doubt the Hereafter, and to beautify sins and desires for them. The four directions were mentioned without above and below because Allah's mercy descends from above, and prostration to Allah takes place in that direction.

As for the difference between the prepositions "min" (from) and "'an" (on/about/away from), "min" indicates the beginning of a purpose or limit, while "'an" indicates deviation or turning away.

Knowledge of the Unseen is exclusive to Allah alone. Iblis did not know the Unseen; rather, he merely assumed and expected that people would follow his whisperings, and his assumption coincided with Allah's decree.