Add link to chapter 4

Author: SamyPesse

README.md

@@ -25,7 +25,7 @@ The goal is to build a very simple UNIX-based operating system in C++, but the g
 
 #### [Chapter 3: First boot with GRUB](Chapter-3/README.md)
 
-#### Chapter 4: Backbone of the OS and C++ runtime
+#### [Chapter 4: Backbone of the OS and C++ runtime](Chapter-4/README.md)
 
 #### Chapter 5: Base classes for managing x86 architecture