what is the difference of the crystal 4 leads and 2 leads? Answered

Ah, that is a different beast.  This circuit uses a crystal oscillator IC, which requires a power source.  Inside, there's a crystal and a small amplifier, which together produce a clock source (for digital electronics, etc).  So the four pins are power, ground, enable and output.

This is a very clever circuit that uses the audio input to modulate the power source of the crystal (or rather, the amplifier inside), thereby generating AM.

So no, you can't use a 2-pin crystal.  If you want to make it just like this circuit, you'll need the four-pin type.  However, if you do a bit more digging I'm sure you'll be able to find an AM transmitter that uses a 2-pin crystal like you've got - it'll just require a few more components.

The two extra leads may be dummy leads that don't do anything; they're just there to support the IC.  What's the part number?  Can you find its datasheet?