SimpleReactRouterUpgrade.js

// In v2/3 you did this:
import ReactDOM from 'react-dom'
import { Router, browserHistory, Route } from 'react-router'

ReactDOM.render(
  <Router>
    <Route path="/about" component={About}/>
    <Route path="/:username" component={User}/>
  </Router>
)

// Those <Route> components above were fake components. They didn't
// really render anything. Instead, we just stripped their props and
// used them as config for the router. In v4, this has changed.

// In v4, your <Route> components actually render to the page! We split
// out the logic that decides which one to render into the <Switch>
// component. Also, we now provide a <BrowserRouter> component you can
// use to configure history management declaratively.
import ReactDOM from 'react-dom'
import { BrowserRouter as Router, Route, Switch } from 'react-router-dom'

ReactDOM.render(
  <Router>
    <Switch>
      <Route path="/about" component={About}/>
      <Route path="/:username" component={User}/>
    </Switch>
  </Router>
)

// There's LOTS more to know (and a lot more advantages to doing things
// this way) but that should get you started! :)