What is the difference between a crystal and one of those 4 pin oscillators...generally?
a crystal control the internal oscillator in the uC whereas a "4 pin oscillator" generates clock pulses that drive the uC.

Generally speaking (my rule of thumb approach) if you do not clock anything but the uC use a crystal, if anything more than the uC is driven by the clock, usa an oscillator.

One exception:
If you need more than ~.1% precision, you need an oven with an oscillator inside to eliminate drift. Maybe someone has come up with another way, I have not heard about it.

Erik