Are you talking about hspa+ or 4g lte in your area? Atrix is only capable of the first and should show an h+ symbol depending on your theme. HSPA+ is att's version of 4g and has 14.4 mb limit on dl speed. 4g lte is a whole other technology requiring a 4g lte radio. Just download speedtest app from the market and you can check your speeds.
Well I have always been confused about the 4G/HSPA+ discussion. AT&T announced today that there is 4G in my area. So what I gather from your response is the Atrix doesn't pick up 4G, only HSPA+, although it is called a Motorola Atrix 4G or AT&T is using 4G loosely to describe their HSPA