Since door/deck speakers are usually 4ohms.. how do I use a 4 channel, or any amp for that matter, that is stable at like 100 x 4 at 2ohms, and get the most power to the speakers.
You can either purchase stronger amp at 4ohms or get speakers that are 2ohms. Some manufacturers will give you a choice. If you have enough space on your doors, you can double up on 4ohm speakers to get 2ohms total (connecting both speakers in parallel).