Air carrier enroute with ZOB initiated a descent reportedly due to four digit same company call sign confusion; reporter suggested company refrain from operating similar sounding call sign flights in the same area at the same time.

Narrative

We were at cruise at FL230. ZOB Center cleared us to FL220. In the descent; we thought ZOB issued a descent to 11;000 FT. The Captain read back the instruction to descend to 11;000 FT. I thought ZOB had said Air Carrier XXXY instead of our flight number of Air Carrier XXXX; but I had not heard any other company planes on the frequency and when the Captain read back the new altitude; there was no correction from ATC. Also the descent clearance seemed to be correct for our location and phase of flight. At about 21;500 FT; in the descent; ZOB asked our assigned altitude; the Captain told him 11;000 FT. ZOB said okay; and gave the current altimeter setting. The Captain asked if there was another company plane on frequency and we were told yes. A couple of minutes later; Air Carrier XXXY checked on and ZOB said they had been looking for them and they must have had a late hand off. The Captain and I decided that he had meant to give clearance to Air Carrier XXXY and we answered for them. Suggestions; I heard Air Carrier XXXY and not Air Carrier XXXX when the clearance to 11;000 was given. I should have asked the Captain to verify the call sign. It was late and the frequency was quiet so I just assumed that it was for us because often ATC will call and the last number in the call sign will be incorrect. I should always get clarification when unsure. Also we should probably not be sending flight with such similar sounding call signs into the same airport at the same time; It creates potential for confusion which it did this night.