Bug

controllers.application.js

import Ember from 'ember';
const { Controller, run } = Ember;

export default Controller.extend({
  appName: 'Ember Twiddle',
  tempValue: '',
  actions: {
    changeValue(event) {
      // prevent default behavior
      event.preventDefault();
      
      let value = event.target.value;
      this.set('tempValue', value);
      
      run.schedule('afterRender', () => {
        //this.set('tempValue', '123');
        this.set('tempValue', value.substring(0, 4));
      });
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<input type="text" value={{tempValue}} oninput={{action "changeValue"}}>
<br>
{{tempValue}}
<br>

twiddle.json

{
  "version": "0.9.3",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-data": "2.6.1",
    "ember-template-compiler": "2.6.0"
  }
}