If you think the first part won't completely make sense without the rest of the book, then finish the whole book first maybe?
The plot is quite standalone and can be read as a seperate work I suppose; but quite a lot of what is discussed and found is only explained in parts 2,3 and 4. And I haven't introduced the final major character or my take on quite a few big ideas.
I had originally imagined the story starting where part 2 is now - but found that to explain the characters and parts of the story I needed a introductory 'prequel' as it was becoming clear that my original idea of having flashbacks was spiralling out of control.
I'm dithering now. I should just stop thinking about it and come back to it fresh!