4. It is possible to have received the Holy Spirit and not realize it.

Reasons why: The receiving of the Holy Ghost is a mystery(John 3:8) and a spiritual occurrence. Thus, it is spiritually discerned (1 Cor 2:14)

It is a supernatural experience that is not dependent on human intelligence for it occur.

Nonetheless, overtime , the Holy Spirit leaves signs indicating his presence for He wants his presence known. So between these events is an interval of oblivion, teaching, and discernment.