AngularFire is an awesome way of connecting the power of Firebase to your Angular application with familiar paradigms. Firebase and AngularFire provide access tools such as a Realtime Database and centralized authentication management, two of the most common problems that developers face.
There were several changes introduced as part of the May 2017 Release Candidate (RC) in 4.0.0-rc.0 designed to improve the speed and performance of AngularFire that change how you use it.

What Happened? AngularFire split into Feature Modules

Before the RC, AngularFire came as a single module with all of the features combined into a single library. This meant that you had to pay the performance and bundle size cost regardless of which Firebase features you were using.

Step 1: Update You Dependencies

You will want to update to the latest version of angularfire2 and of firebase. This is pretty straightforward whether you are using npm or yarn. Here's the yarn command:
yarn add angularfire2@latest firebase@latest

Step 2: Update your NgModule

We need to update the way we import AngularFire and add the module for Database and the module for Auth as well. If you used your NgModule to preconfigure authentication methods & providers, you can't do this anymore. These are now just passed in with your login method.
Before: 
import { AngularFireModule } from 'angularfire2';
...
imports: [
    ...
    AngularFireModule.initializeApp( FIREBASE_CONFIG_OBJECT),
]

After: 
import { AngularFireModule } from 'angularfire2';
import { AngularFireDatabaseModule } from 'angularfire2/database';
import { AngularFireAuthModule } from 'angularfire2/auth';
...
imports: [
    ...
    AngularFireModule.initializeApp( FIREBASE_CONFIG_OBJECT),
    AngularFireDatabaseModule,
    AngularFireAuthModule,
]

Step 3: Update your Services

Anywhere you injected AngularFire, you now want to inject AngularFireDatabase or AngularFireAuth as needed.
constructor(af: AngularFire) { data = af.db.get('/path') } becomes constructor(db: AngularFireDatabase) { data = db.get('/path') }
and
constructor(af: AngularFire) { user = af.auth; } becomes constructor(auth: AngularFireAuth} { user = auth.authState; }
Any imports for FirebaseListObservable or FirebaseObjectObservable have also been moved to angularfire2/database.

Step 4: Update your auth method calls

Almost everything is the same when it comes to Database access and usage, but a couple of the auth methods have changed.
Before: 
this.af.auth.login({
    provider: AuthProviders.Google,
    method: AuthMethods.Popup,
});

After: 
import * as firebase from 'firebase/app';
...
this.auth.auth.signInWithPopup(new firebase.auth.GoogleAuthProvider());

Summary

That should be it. You should now be running the latest and greatest AngularFire, and only using the Firebase libraries and dependencies where and when you need them in your application.