They'll work, and as long as they're the same brand, you shouldn't really have any issues. It IS possible that an unmatched pair will cause problems, but you should be ok.
Question though. Usually a single module is just half of what a Dual Channel kit costs, like this
G.SKILL Ripjaws Series 4GB 240-Pin DDR3 SDRAM DDR3 1333
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231310 $21.99
G.SKILL Ripjaws Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1333
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231311 $43.99
What RAM were you looking at that was any different?