<Switch>
    <Router path="/users/:uid" component={Profile} />
    <Router path="*" component={NotFound} />
</Switch>

app.get('/users/:uid', (req, res) {
    if( // user exist) {
        res.json({user})
    }

    else res.status(404).json({ message: 'User not exist'})
})

class Profile extends Component {

   componentWillMount = () => {
       this.props.fetchUser(uid)
   }

   render(){

       if(!this.props.loading && this.props.user) return <NotFound />
       else return (...)
   }
}