As everybody has stated, it's best to use the correct impedance that the amplifier is rated for. Use 4 ohm speakers if the amplifier was rated for 4 ohms.
What are you trying to do? Do you have 2 ohm speakers lying around and just want to make use of them? Are you considering a specific brand and model of speaker that is only available as 2 ohm loads (ex. current Infiinity Kappas)? Are you trying to get more SPL without changing the amplifier?
If you want to get the most SPL from a given power output, use a speaker that has the highest sensitivity rating you can find. For example, the current range Infiinty Reference speakers are rated at approx 92dB. The current range of Infinity Kappas are rated at 94-95dB. That is, with the same voltage level (which equates to a volume setting), the Kappa will play louder than the Reference. 3dB is noticecably louder, but not twice as loud (human hearing is not linear). Still, looking at it another way, to get the Reference speaker to play at the same level as the Kappa, the Reference speaker would have to draw double the power that the Kappa would.
Unfortunately the Kappas are 2 ohm loads, vs 4 ohms for the Reference, so not really usable in your case, unless you upgrade to an amp that can handle 2 ohm loads. And remember, the above is only one of many factors when considering the purchase of a speaker. Physical compatibility (will it fit), sound quality, tonal neutrality, etc, all have to be considered to.
The logical process would be to find speakers that sound the way you like, can be made to fit into the car, and then get an amplifier that will drive them to the volume level you desire. But, if you have different priorities, then the process has to be adapted to fit your requirements. I don't listen to my music that loud, and I wanted to do as little custom mods as possible while still getting some improvement in sound quality, so I experimented with just replacing the stock speakers and applying sound deadener to the doors. I went the opposite direction that you're proposing; I have the Bose system which has 2 ohm speakers. I put in 4 ohm Infinity Reference (I prefer the highs of the sof dome tweeter) speakers in all 4 doors, driven off the stock Bose amp. I have to turn the volume control up higher than with stock (probably due to the References being less efficient, ie lower sensitivity, than the stock speakers), but it still plays loud enough for me. The setup meets my needs for now; the remaining question is whether I should replace the Bose sub.