There is no mention of Illustrator in Part 2 so I assume you mean Part 3?
If you've managed that Ok then there is nothing different in Part 4. As the tutorial states, "Open your map in Illustrator and start tracing the borders with the pen tool..."
There isn't ANY need whatsoever to use Illustrator for ANY of this tutorial, it can all be done with PS, and far easier than is demonstrated but I see no reason why you cannot apply what you have already done "with ease" to this next step.
There are no hard and fast 'rules' when it comes to software and many times there are always more than one way to do things. In this case the author has, in my eyes, complicated the project, but I'm sure they have their reasons....probably because they don't know how use Photoshop properly.
You seem like you have an ample understanding of the software involved so its hard to believe you cannot apply it further to the task in hand.
In a nutshell you have 'areas', (states), which are copied from a template, (the map), to their own layer, coloured, (fill), and have a border, (stroke). Water is exactly the same but with a 'glow' and a pattern overlay.
Each 'state' would need to be on its own layer in order for the 'fill' and 'stroke' to work.
Water should also be on its own layer.
Rivers too should be on a layer unless they need different properties.
Text will create its own layer automatically.
Honestly, there is nothing difficult about this at all, its ALL very basic Photoshop.
(Unless I'm missing something....which is more than likely.)
Regards.
MrToM.
