A shining silver sphere appeared

I have been only four years old and is very strange how do I remember such a thing. I don't recall the exact date except for the year counting the time I used to live in that house. 
Every time I went to bed a shining silver sphere smaller than a football ball appeared.  I can recall the position of this sphere as I was laying on the bed and this ball came from the right side passing through the wall of the room.  Its speed was slow.  It only observed and left passing through the other wall on the left side.  It happened to come back where it came from in the same way.  I was scared and frozen, half awake and half sleepy.  
I can say that I think about this many times and sometimes think that this was only a dream, but how is possible that a four years old child see such a thing and why do I still remember. 
Now I am 27 years old.
Date in 1979 is approximate.  PD