react-router4.0实现重定向与404功能

import React, { Component } from 'react';
import axios from 'axios';
import { Redirect } from 'react-router-dom';
class Login extends Component{
 constructor(){
  super();
  this.state = {value: '', logined: false};
  this.handleChange = this.handleChange.bind(this);
  this.toLogin = this.toLogin.bind(this);
 }
 handleChange(event) {
  this.setState({value: event.target.value})
 }
 toLogin(accesstoken) {
  axios.post('yoururl',{accesstoken})
   .then((res) => {
    this.setState({logined: true});
   })
 }
 render() {
  if(this.props.logined) {
   return (
    <Redirect to="/user"/>
   )
  }
  return (
   <p>
     <input type="text" value={this.state.value} onChange={this.handleChange} />
     <a onClick={() => { this.toLogin(this.state.value) }}>登录</a>
   </p>
  )
 }
}
export default Login;

const LOGIN_SUCCESS = 'LOGIN_SUCCESS';
const LOGIN_FAILED = 'LOGIN_FAILED';
const LOGINOUT = 'LOGINOUT';
const Login = function(state, action) {
 if(!state) {
  console.log('state');
  if(sessionStorage.getItem('usermsg')) {
   return {
    logined: true,
    usermsg: JSON.parse(sessionStorage.getItem('usermsg'))
   }
  }
  return {
   logined: false,
   usermsg: {}
  }
 }
 switch(action.type) {
  case LOGIN_SUCCESS:
   return {logined: true,usermsg: action.usermsg};
  case LOGIN_FAILED:
   return {logined: false,usermsg:{}};
  case LOGINOUT:
   return {logined: false, usermsg: {}};
  default:
   return state
 }
};
export default Login;

<Switch>
 <Route path="/" exact component={Home}/>
 <Route path="/user" component={User}/>
 <Route component={NoMatch}/>
</Switch>