Those are the four that matter. Become an expert in HTML, CSS, JavaScript, and PHP. I would study them in that order.
The first three are client side languages used for presentation and surface manipulation of data. PHP is the server side language that WP and most modern web applications are written in. It crunches the user selections and data from the client side, interacts with the database, and determines what html, css and JS to serve in response.
The other big piece of the puzzle is the MySQL database. Other database structures exist, but this is the most popular today. A thorough understanding of how PHP interacts with a MySQL database is absolutely necessary to contribute to WP or develop code for any PHP based application. I studied and practiced for about 5 years before I started taking on client work. The good news is, you can be completely self taught thanks to the great resources on the web.
The best way to get started, is to build yourself a site that advertises your services and learn during the process. Reading and taking tutorials helps a lot at first, but quickly you need actual real world goals to work towards. Participating on forums and answering questions is a great way to learn. Its constant real world examples of problems that users are actually having. If you can solve their problems then you will be building your skills to develop your own applications. Sometimes the issue takes a lot of investigation, delving into the code to find the answer.
Choosing a platform like WP, Drupal, Joomla, X-cart, or Magento to use as your learning base will help you immensely. Become an expert in one or two platforms, one a CMS and the other e-commerce. Then you will have a well rounded skill set for any website building project.
