I want to mount a scope on my flat top M4. Is there a mount I can use that will allow me to keep my front iron sight? I saw a picture in a magazine of a guy with this set up. it looked like it was a quick release set up.

     There's plenty of room to mount a scope without removing the front sight; above 4-6x or so, you won't even see a shadow.  You'll want to use a setup that mounts optics at the same height as irons to maintain a constant position on the rifle; that typically requires extra-high rings, a riser with standard-height rings, or a dedicated one-piece mount.

    Be sure to consider eye relief as well as height - the Picatinny rail on a flat-top may not let you mount the scope far enough forward for a comfortable position, especially in prone. I use an extended riser (1/2" high) made by Accuracy Speaks (available direct or from Brownell's) combined with standard-height rings.  There are now a similar ones available from Compass Lake and White Oak, among others.  This gives me an extra 4" of rail to let me mount a Leupold 6.5-20x50 LRT (a rather long scope), and gives me the option for an extra 15' of built-in elevation for long range work.  For ranges up to 600 yards, a simple rail extender like YHM's <http://www.yhm.net/store/rails.html> should do the job for less money.

   I'm using Burris Signature Zee rings - they are inexpensive and work well, even though they don't have the macho look many seem to like.  They use a floating plastic bushing to minimize stress on the scope tube and simplify installation, and offset bushings are available.  Bushings with a 0.010" offset are included with 30 mm rings, and 0.010", 0.020" and 0.030" bushings can be purchased for 1" rings.  They're inexpensive and work very well.  I use them on my spacegun to mount a VX-III 6.5-20x50 LRT and have never had a problem with them.  Removing and replacing the scope has never shifted the POI.

   Also remember that your windage range is restricted when you are at maximum or minimum internal elevation.  You can't apply maximum elevation and windage simultaneously due to the round scope tube.  You won't need large amounts of windage at 100 yards, but you could need 20+ minutes at 1000 on a windy day.  There are minor optical distortions that may be more apparent off center, but the biggest reason to stay on center at long range is to have adequate windage adjustment available.