2 ohm vs. 4ohm

The ohm-rating is simply the average impedance of the voice coil(s).  There are voice coils available out there anywhere from less than one ohm to 40 or 50 ohms.  Your speaker will not sound any different if you wire it for 2 ohms or (I suspect you mean) for 8 ohms.  It will, however, place a very different load on your amplifier. 

Amplifiers produce voltage which allows current to flow through whatever is connected to them.  Review Ohm's Law (link to the left.)  If an amplifier outputs, say, 10 volts and a 2-ohm impedance load is connected to it, it will draw nominally 5 amps of current producing approximately 50 watts.  If the amp outputting 10 volts has 8 ohms connected to it, only 1.25 amps of current will flow, producing approximately 12.5 watts.  Current flow produces heat, so the 8-ohm load will heat up far more slowly than the 2-ohm load, and the amplifier will also not get as hot so it will last far longer, produce less noise and distortion, and produce a cleaner better sounding signal for the speaker.  So the higher impedance load generally "sounds better" but only because it places less load and strain on the amplifier.

By the way, to get 50 watts into the 8-ohm load, you simply need a larger amplifier capable of producing more voltage.

The amplifier will remain stable at any ohm load from 2 ohms up.  You could put speakers with a 3 ohm impedance onto it, or speakers at 6 ohms.  It is the impedance "load" that makes the amplifier provide power.  It is a natural electrical occurrence, not a special feature of audio amplifiers.  Too low of a load, say in this case 1 ohm, would cause the amplifier to produce too much current for its own stability.  It would burn out or quit working.  A load at 2 ohms or higher would allow the amplifier to function.

The lowest load that can safely be put on an amp will net the most power at the expense (usually) of sound quality.  A higher load will net less output but the quality of the amp's production will be improved.  That's why the front speakers shouldn't be wired so that they make the amplifier work at lower than a 4 ohm load;  the lower sound quality would be apparent.  With the low frequencies of subwoofers, however, the better quality amplifiers will produce output at the lower loads that will sound good enough for most people.

Look over the spec output ratings of that amp and others.  You'll readily see the differences in power output at the specified load conditions.  But note that the quality-related specs are shown as tested under 4 ohm load conditions, not lower.  The lower loads would produce worse specs (like harmonic distortion, damping factor, etc.).

This is a good question you ask because it is the first thing a person needs to understand before selecting an amplifier / speaker combination.  That is, how the speakers will combine or wire together to create the ohm load you want to use with a specific amplifier.  SQ guys will stick with 4 ohm (or higher) even with subs.