Lots of finch eggs but no chicks?

I acquired four zebra finches a little while ago. I placed a nest in the cage to see if I could increase the population. I often find eggs on the floor of the cage and when I run the sometimes up to three finches out of the nest I find up to 10 eggs being sat on. Is it my luck and received four females? How do I tell if I have females or males? Should I buy another finch and take the word of the pet shop person and hope it's a male?