react router v4 default page(not found page)

React Router’s No Match documentation covers this. You need to import the <Switch> component, then you can remove the path attribute altogether.

A <Switch> renders the first child <Route> that matches. A <Route> with no path always matches

This is the example that uses:

<Router>
  <div>
    <Switch>
      <Route path="https://stackoverflow.com/" exact component={Home}/>
      <Redirect from="/old-match" to="/will-match"/>
      <Route path="/will-match" component={WillMatch}/>
      <Route component={NoMatch}/>
    </Switch>
  </div>
</Router>

So in your case, you’d simply drop the path="*" and introduce the <Switch>:

<Switch>
  <Route exact path="https://stackoverflow.com/" component={Home}/>
  <Route path="/user" component={User}/>
  <Route component={Notfound} />
</Switch>

Remember to include Switch to your import statement at the top.