This tutorial has no GT4 prerequisite documents, since it is intended as a starting point for GT4 programmers. However, you should already be familiar with Grid Computing. A good, short introduction to what Grid Computing is can be found in Ian Foster's paper The Grid: A New Infrastructure for 21st Century Science. A more extense, and very easy to read, introduction can be found at the Grid Café.

For a much more detailed text, you might want to check out the following book: The Grid 2: Blueprint for a New Computing Infrastructure (Edited by Ian Foster and Carl Kesselman. 2003). Most of the book is easy to read and not too technical. It is also known as "The Grid Bible". With a name like that, you can assume it's worth taking a look at it :-)

You might also be interested in taking a look at the 'Publications' section in the Globus website, specially the documents listed below. However, these documents are rather technical and might be too hard for a beginner. You might want to just skim through them at first, and then reread them once you're familiar with GT4.