Update theme version from v3 to v4 in README

Author: CaiJimmy

README.md

@@ -36,11 +36,11 @@ In case you don't want to use GitHub codespace, you can also run this template i
 Run:
 
 ```bash
-hugo mod get -u github.com/CaiJimmy/hugo-theme-stack/v3
+hugo mod get -u github.com/CaiJimmy/hugo-theme-stack/v4
 hugo mod tidy
 ```
 
-> This starter template has been configured with `v3` version of theme. Due to the limitation of Go module, once the `v4` or up version of theme is released, you need to update the theme manually. (Modifying `config/module.toml` file)
+> This starter template has been configured with `v4` version of theme. Due to the limitation of Go module, once the `v4` or up version of theme is released, you need to update the theme manually. (Modifying `config/module.toml` file)
 
 ## Deploy to another static page hostings