Hi Jecksons,

The 4-pin oscillator you're refering to is a standalone circuit that has a crystal in it. That's why it needs Vcc and GND. When powered up, it energizes a crystal and produces an output signal (relative to ground). That signal is then applied to one of the XTAL pins of your µ-controller and serves as the clock.

If you choose to use a crystal instead of an oscillator, then you must connect both pins of the crystal to both of the XTAL pins on your µ-controller, along with a couple of capacitors. The internal circuitry of the µ-controller will then stimulate the crystal (assuming you've chosen your capacitors well) and produce the clock signal on the necessary pin. Refer to your documentation for capacitor values.

Joe