4-pin quartz resonators - shall the 2 ground pins be connected to ground plane of the pcb?
Yes, you need to connect pins 2 and 4 to the ground plane of the PCB. It is better to add a ground fill around the device and add guard traces also so that the clock frequency generated gets better immunity from noise. 

However, you should also check the clock layout considerations of your target device to see if there are any more constraints which you need to take care of. 

For starters, you can refer to this link (http://www.atmel.in/Images/doc8128.pdf) which tells about the general clock layout considerations.