I want purchase a 4 channel amp for my front and rear speakers. My deck is 4ohm and the speakers are 2ohm.

So hypothetical question. If I buy an amp that is 2ohm @ 100x4 and 4ohm @ 50x4. how is this going to work out in the end?

 

deck has nothing to do with it as the RCA connections will be the connection from the deck to the amp.

 

This would only matter if your deck was powering your speakers directly. You will likely use the high-level (old speaker wires) or low-level (RCA Cables) to feed your amp and then the amp will feed the speakers. It is the amp-speaker route that you need to worry about. If you buy that amp, it will do 100W to each of those speakers at 2ohm if the amp lives up to the specs (some do and some don't, depends on the amp).

If you ran the speakers with the deck, then you could have problems. The lower resistance from the speakers could get too much power from the deck and it could have issues. It is also possible that the deck will work at 2 ohms, but I don't know without seeing the specs.

I you post the amp model, brand, etc. and the speaker brand/model, then you might get some opinions/experience before you buy.