I have two girls. One is 4 and the other 2. When my 4 year old started prek, my 2 year old missed her a great deal and immediately came the imaginary friends. She started calling "friends names" and actually playing for long periods of time. She was so into it that she started waking up during the night to continue the play time with them. She still does this at about 2 or 3 AM she is talks loud and laughs hysterical just playing. I didn't have this with my 4 year old so it's a new experience for me and different for sure. It is amazing how much she has. I wonder if it is loneliness that triggers this (from missing her sister while she is in school).