Hi there!
To target countries the best way to go is with ccTLDs and if you don't have the ccTLDs then it is with subdirectories within a generic TLD, so you'll have:
- For the US: yourbrand.com/category-a/
- For the UK: yourbrand.com/en-gb/category-a/
- For Spain: yourbrand.com/es-es/categoria-a/
- For Mexico: yourbrand.com/es-mx/categoria-a/
Please keep in mind that the "name" of the directory here it's not important but is just to keep it usable and short and follow the language naming conventions. What it is important is that each country has its own consistent directory structure.
It's fundamental that the look and feel as well as all of the elements of each ones of your different country versions (in the different directories) are localized to target their audience: From the translation of the URLs, titles, descriptions, headings, text, etc. to the appropriate language, using the right currency, etc.
In order to geolocate each directory and inform Google that they're targeting different countries you can do it through Google Webmaster Tools with the "Geolocate" option by registering each directory independently and targeting it.
Additionally, if you have many country versions with the same language (US & UK or Spain & Mexico), in order to avoid having content duplication issues, informing Google that each one of these pages are in these languages but targeting different audiences you should use the hreflang tags as specified here.
By doing this you'll make sure you'll have the base set to target your different country search audiences with Google without running into content duplication issues.
For more information about how to establish and identify the best strategy to follow take a look at this post I wrote some time ago about International SEO strategy.
If you have any question just let me know!
