Add link to 4th and final part of the tour. (#133)

* Add link to 4th and final part of the tour.

* Update README.md

Author: mbrandonw

README.md

@@ -41,7 +41,7 @@ This library provides a few core tools that can be used to build applications of
 
 The Composable Architecture was designed over the course of many episodes on [Point-Free](https://www.pointfree.co), a video series exploring functional programming and the Swift language, hosted by [Brandon Williams](https://twitter.com/mbrandonw) and [Stephen Celis](https://twitter.com/stephencelis).
 
-You can watch all of the episodes [here](https://www.pointfree.co/collections/composable-architecture), as well as a dedicated, multipart tour of the architecture from scratch: [part 1](https://www.pointfree.co/episodes/ep100-a-tour-of-the-composable-architecture-part-1), [part 2](https://www.pointfree.co/episodes/ep101-a-tour-of-the-composable-architecture-part-2) and [part 3](https://www.pointfree.co/episodes/ep102-a-tour-of-the-composable-architecture-part-3).
+You can watch all of the episodes [here](https://www.pointfree.co/collections/composable-architecture), as well as a dedicated, multipart tour of the architecture from scratch: [part 1](https://www.pointfree.co/collections/composable-architecture/a-tour-of-the-composable-architecture/ep100-a-tour-of-the-composable-architecture-part-1), [part 2](https://www.pointfree.co/collections/composable-architecture/a-tour-of-the-composable-architecture/ep101-a-tour-of-the-composable-architecture-part-2), [part 3](https://www.pointfree.co/collections/composable-architecture/a-tour-of-the-composable-architecture/ep102-a-tour-of-the-composable-architecture-part-3) and [part 4](https://www.pointfree.co/collections/composable-architecture/a-tour-of-the-composable-architecture/ep103-a-tour-of-the-composable-architecture-part-4).
 
 <a href="https://www.pointfree.co/collections/composable-architecture">
   <img alt="video poster image" src="https://i.vimeocdn.com/video/850265054.jpg" width="600">