Consider the following statements regarding the military strategies employed during the Delhi Sultanate:

1. Alauddin Khalji established an efficient espionage system to prevent rebellion.
2. Muhammad bin Tughlaq’s military campaigns in South India were largely unsuccessful and led to economic strain.
3. Iltutmish established a well-organized standing army.
4. The Sayyids relied extensively on mercenaries for their campaigns in northern India.

Which statements are correct?