4 or 2 ohms on a 4channel?

This might help to clarify it:

• 4-channel amplifier = four "mono" amplifiers.
• Any amplifier can be used for subwoofers as well as other speakers.
• The load is the impedance of the speaker(s) attached to the amplifier.
• An amplifier has a limit to the amount of current it will produce.  "Stable at 2 ohms" means that a load lower than 2 ohms will cause the amplifier to produce current greater than its capacity to handle it successfully.
• A lower impedance load results in lower voltage / higher current from the amplifier.  A higher load results in higher voltage / lower current.  See the minimum load limitations in the amp's specs.

(An amplifier is specific in its specs about the lowest impedance load it can handle, but it will always be able to handle a load with higher impedance.  Some amps will spec out numbers for 4 ohms, some will go as high as 8 ohms in the specs.)

Wiring a sub to a 4 channel amp:  most commonly bridged across two of the channels.  When an amplifer is stable at 2 ohms, it means 2 ohms per channel.  Bridging is combining two channels, so when you do that you have to be sure the load is 4 ohms or higher because the load will be split between the two channels.  A 4 ohm load bridged is equivalent to 2 ohms/channel.

Where you would put a 2 ohm load onto a mono sub amp, you would put a 4 ohm load bridged onto a two-channel amp.  The output of each amp would be at 2 ohms.

• Meet Mr. Ohm's Law:

........where power is the product of voltage times current, and the impedance of the load will determine the ratio.

A great study is chapters 2 - 12 of http://www.bcae1.com/

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.