4D Client
You could (obviously) use the native 4D Client to connect to 4D Server.
4D ODBC
You could use ODBC to connect to 4D Server from a generic client.
You would need to install the 4D ODBC Driver appropriate to your environment on the client machine. Remember to match the version of ODBC Driver to the version of 4D and the architecture of the ODBC Driver with the architecture of the ODBC Client.
For example, if your 4D Server is running v15.2 and the generic client is running in 64 bit mode, you would then want to use the 4D v15.2 ODBC Driver 64 bit
On the other hand if 4D Server is running 14.5 and the generic client is running in 32 bit mode, you would want to install the 4D v14.5 ODBC Driver 32 bit
The ODBC Drivers can be downloaded from http://4d.com/downloads/products.html
4D Native SQL Protocol
The 4D SQL Protocol is documented at Github - 4D/SQLlib_4D and also on sources.4d.com so theoretically you could write a native connector of some sort.