diff --git a/CHANGELOG.md b/CHANGELOG.md
index 508de030..9fa510d9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,14 +2,62 @@
 
 ### 4.0.0
 
-* Introduced `useStaticRendering(boolean)` to better support server-side rendering scenerios. See [#140](https://github.com/mobxjs/mobx-react/issues/140)
-* Introduced `Observer`. Can be used as alternative to the `observer` decorator. Marks a component region as reactiove. See the Readme / [#138](https://github.com/mobxjs/mobx-react/issues/138)
+#### `inject(fn, component)` will now track `fn` as well
+
+`inject(func)` is now reactive as well, that means that transformation in the selector function will be tracked, see [#111](https://github.com/mobxjs/mobx-react/issues/111)
+
+```javascript
+const NameDisplayer = ({ name }) => <h1>{name}</h1>
+
+const UserNameDisplayer = inject(
+    stores => ({
+        name: stores.userStore.name
+    }),
+    NameDisplayer
+)
+
+const user = mobx.observable({
+    name: "Noa"
+})
+
+const App = () => (
+    <Provider userStore={user}>
+        <UserNameDisplayer />
+    </Provider>
+)
+
+ReactDOM.render(<App />, document.body)
+```
+
+#### Better support for Server Side Rendering
+
+Introduced `useStaticRendering(boolean)` to better support server-side rendering scenerios. See [#140](https://github.com/mobxjs/mobx-react/issues/140)
+
+#### Introduced `Observer` as alternative syntax to the `observer` decorator.
+
+_This feature is still experimental and might change in the next minor release, or be deprecated_
+
+Introduced `Observer`. Can be used as alternative to the `observer` decorator. Marks a component region as reactiove. See the Readme / [#138](https://github.com/mobxjs/mobx-react/issues/138)
+Example:
+
+```javascript
+const UserNameDisplayer = ({ user }) => (
+    <Observer>
+        {() => <div>{user.name}</div>}
+    </Observer>
+)
+```
+
+#### Other improvements
+
 * Clean up data subscriptions if an error is thrown by an `observer` component, see [#134](https://github.com/mobxjs/mobx-react/pull/134) by @andykog
 * Print warning when `inject` and `observer` are used in the wrong order, see #146, by @delaetthomas
 * export `PropTypes` as well, fixes #153
 * Add react as a peer dependency
 * Added minified browser build: `index.min.js`, fixes #147
 
+---
+
 ### 3.5.8
 
 * Fixed issue where `props` where not passed properly to components in very rare cases. Also fixed #115
diff --git a/README.md b/README.md
index 8c884944..20f584a6 100644
--- a/README.md
+++ b/README.md
@@ -78,6 +78,8 @@ const TodoView = observer(({todo}) => <div>{todo.title}</div>)
 
 ### `Observer`
 
+_This feature is still experimental and might change in the next minor release, or be deprecated_
+
 `Observer` is a React component, which applies `observer` to an unanymous region in your component.
 It takes as children a single, argumentless function which should return exactly one React component.
 The rendering in the function will be tracked and automatically be re-rendered when needed.
@@ -254,7 +256,40 @@ class MyComponent extends React.Component {
 }
 ```
 
+#### Customizing inject
+
+Instead of passing a list of store names, it is also possible to create a custom mapper function and pass it to inject.
+The mapper function receives all stores as argument, the properties with which the components are invoked and the context, and should produce a new set of properties,
+that are mapped into the original:
+
+`mapperFunction: (allStores, props, context) => additionalProps`
+
+Since version 4.0 the `mapperFunction` itself is tracked as well, so it is possible to do things like:
+
+```javascript
+const NameDisplayer = ({ name }) => <h1>{name}</h1>
+
+const UserNameDisplayer = inject(
+    stores => ({
+        name: stores.userStore.name
+    }),
+    NameDisplayer
+)
+
+const user = mobx.observable({
+    name: "Noa"
+})
+
+const App = () => (
+    <Provider userStore={user}>
+        <UserNameDisplayer />
+    </Provider>
+)
+
+ReactDOM.render(<App />, document.body)
+```
 
+_N.B. note that in this *specific* case neither `NameDisplayer` or `UserNameDisplayer` doesn't need to be decorated with `observer`, since the observable dereferencing is done in the mapper function_
 
 #### Strongly typing inject
 
diff --git a/package.json b/package.json
index a566445a..8f10e640 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "mobx-react",
-  "version": "3.5.9",
+  "version": "4.0.0-beta.2",
   "description": "React bindings for MobX. Create fully reactive components.",
   "main": "index.js",
   "typings": "index",
@@ -56,4 +56,4 @@
     "reactjs",
     "reactive"
   ]
-}
\ No newline at end of file
+}
diff --git a/src/inject.js b/src/inject.js
index fc0bc203..2eac2319 100644
--- a/src/inject.js
+++ b/src/inject.js
@@ -1,5 +1,6 @@
 import React, { PropTypes } from 'react';
 import hoistStatics from 'hoist-non-react-statics';
+import {observer} from './observer';
 
 /**
  * Store Injection
@@ -22,7 +23,6 @@ function createStoreInjector(grabStoresFn, component) {
 
       return React.createElement(component, newProps);
     }
-    // TODO: should have shouldComponentUpdate?
   });
 
   Injector.isInjector = true;
@@ -77,14 +77,19 @@ export default function inject(/* fn(stores, nextProps) or ...storeNames */) {
   let grabStoresFn;
   if (typeof arguments[0] === "function") {
     grabStoresFn = arguments[0];
+    return function(componentClass) {
+      // mark the Injector as observer, to make it react to expressions in `grabStoresFn`,
+      // see #111
+      return observer(createStoreInjector(grabStoresFn, componentClass));
+    };
   } else {
     const storesNames = [];
     for (let i = 0; i < arguments.length; i++)
       storesNames[i] = arguments[i];
     grabStoresFn = grabStoresByName(storesNames);
+    return function(componentClass) {
+      return createStoreInjector(grabStoresFn, componentClass);
+    };
   }
-  return function(componentClass) {
-    return createStoreInjector(grabStoresFn, componentClass);
-  };
 }
 
diff --git a/test/inject.js b/test/inject.js
index 05692b64..b5dd85e1 100644
--- a/test/inject.js
+++ b/test/inject.js
@@ -327,5 +327,21 @@ test('inject based context', t => {
         t.end();
     })
 
+    test('using a custom injector is reactive', t => {
+        const user = mobx.observable({ name: "Noa"})
+        const mapper = (stores) => ({ name: stores.user.name })
+
+        const DisplayName = (props) => e("h1", {}, props.name)
+        const User = inject(mapper)(DisplayName)
+        const App = () => e(Provider, { user: user }, e(User, {}))
+
+        const wrapper = mount(e(App))
+        t.equal(wrapper.find("h1").text(), "Noa")
+
+        user.name = "Veria"
+        t.equal(wrapper.find("h1").text(), "Veria")
+        t.end()
+    })
+
     t.end()
 })
\ No newline at end of file