I am trying to build an application that pulls data from a 4D database to fill in form options. Then allow the user to select from those options and submit the form. I am not to familiar with 4D so any advice would be appreciated.
You have multiple options:
PDO. You can use PHP Data Objects (PDO) to connect to 4D via PDO_4D, this uses the 4D SQL protocol. PDO_4D is discussed on the PHP website and on 4D's Knowledge Base.There are a number of forks on Github that include additional bug fixes, see here.
ODBC. If the PHP is running on Windows or Mac and you prefer ODBC over PDO, then you could use the official 4D ODBC Drivers (available for Mac/Win) on the machine running PHP and then use PHP's ODBC commands.
REST. You can write a method in 4D that is accessible via the web and returns a JSON object containing the data you need. Then from your page you can call that REST service to obtain the values. You could use JavaScript to make the call via XHR or you can use PHP to access the REST service.
SOAP. You can write a method in 4D and make the method available via SOAP. Then use a SOAP client in PHP to interact with the soap service.