After all the trial and error I had done with trying to create my gradient I decided to do some research into creating four point gradients and finally came across a method using Adobe Illustrator. Using the mesh tool on a rectangle in illustrator I was finally able to make the four point gradient that I wanted. Rather than sticking to these four colours I found that I could make multiple gradients that could fit together using more colours.
After testing and playing around with different colours, this is what i finally came up with. I then went through the process of using the colour picker tool to select individual colours on the gradient to form the colours for my book.