How to Pass argument from URL to field in Views
This is done using contextual filters. The easiest way to do it is to set up a "page" display by clicking Add+ on the views page. Give the page the URL of http://[path]/event/% (you may also have to try %.json depending on the exact call you are trying to make, the json files are known to drupal right?). The % gives you the argument you are looking for. Then you need to set up a Contextual Filter found under the advanced section of the view. Here you pick the field you are passing the number 4 or 4.json to and choose the correct settings. You may find that you need to pick Provide Default Value for this exact case and then use raw value from url or even php code and do some magic on the $argument (depending on exactly what it is you are doing).