https://www.udemy.com/course-dashboard-redirect/?course_id=1134390 -- Lecture 31 (React Router v4) seems to be a little behind react-router-dom 4.2.2. Here's my fix

Four.js

import React, { Component } from 'react';
import { Route, Link } from 'react-router-dom';

import Fourpointone from './Fourpointone';

class Four extends Component {
  render() {
    return (
      <div>
        <h1>I am a Four component</h1>
        <Link to="/Four/123">Four Point One</Link>
        {/* Define the children in here now, as per:
            https://github.com/ReactTraining/react-router/blob/master/packages/react-router/docs/guides/migrating.md#nesting-routes
        */}
        {/* NOTE: The position of the 'Route' is important, as it determines where the child component actually loads */}
        <Route path="/Four/:id" component={Fourpointone} />
      </div>
    );
  }
}

export default Four;

Fourpointone.js

import React, { Component } from 'react';

class Fourpointone extends Component {
  render() {
    return (
      <div>
        <h2>I am a FourOne component</h2>
      </div>
    );
  }
}

export default Fourpointone;

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import registerServiceWorker from './registerServiceWorker';

// Had to use 'react-router-dom' as 'react-router' was missing certain elements.
import {
  BrowserRouter as Router,
  Switch,
  Route,
} from 'react-router-dom';

import App from './App';
import One from './One';
import Two from './Two';
import Three from './Three';
import Four from './Four';
import NoMatch from './NoMatch';

ReactDOM.render(
  <Router>
  {/* No history as it seemed to be implicit now. */}
    <div>
      <Switch>
        <Route exact path="/" component={App}></Route>
        <Route path="/One" component={One}></Route>
        <Route path="/Two" component={Two}></Route>
        <Route path="/Three" component={Three}></Route>
        {/* The children should no longer be defined here */}
        <Route path="/Four" component={Four} />
        <Route component={NoMatch}></Route>
      </Switch>
    </div>
  </Router>
  ,
  document.getElementById('root'));
registerServiceWorker();