You said you wanted to create a "template" for each of the 4 categories. Creating a page template may be a little more technical than you're ready for if you're just getting started with wordpress, but that depends on your coding knowledge. How comfortable are you with HTML, CSS, & PHP?
Also, if you're interested in creating real page templates to use with your theme, it's a good idea to learn how to create a child theme and add them that way. Child themes allow you to customize your theme without modifying any parent theme files. You can find a good introduction to child theme basics here.
In addition to that, another solution to creating the structure you want would be using pages and custom menus like this:
- Parent Category Page
- child page 1
- child page 2
- child page 3
- child page 4
- child page 5
You would then need to create the navigation menu you want to show up in the sidebar in Appearance > Menus. Create 1 menu for each category, then use the Custom Menu widget to add each menu to the sidebar. Then you just have to set each one to only display on the pages where it should show up. Install and activate the Display Widgets plugin. This allows you to choose what page to show/hide each widget on. Once activated, in the admin Widgets page you will see on each widget in a sidebar has an added section (once it's expanded) where you can set the Display Widgets options.
This method is a little more complicated and will take a little more time to setup than the first answer, but it will make customizing your pages easier as not all themes have different layout options for posts.
Hope that helps!