使用koa-views时访问404

app.js代码如下

'use strict';

const Koa = require('koa');
const app = new Koa();

const views = require('koa-views');

app.use(views(__dirname + '/views', {
    extension: 'hbs',
    map: { hbs: 'handlebars' }
}));

app.use((ctx, next) => {
    return next().then(function () {
        ctx.render('index');
    });
});

app.listen(4000);

希望各位大大帮忙解惑