What would be a good sentence to describe Prime Meridian to a fourth grader?
You can read more about it here.
http://en.wikipedia.org/wiki/Prime_meridian
I would think you could say a meridian is a group of vertical lines drawn on the surface of a sphere that divides the sphere into sections. The sections are numbered from zero to 360 for a sphere. The PRIME meridian is the vertical line at zero.
It might help in fourth grade if you had a globe with the lines on it and you could point them out at zero, 10, 20 etc. A tennis ball with lines drawn in ink might help also.