I have a Colt M4 flat top upper and was wondering, to use a Sheppard scope will I need a riser to clear the front sight or will the standard rings raise it enough.

If you are using a 4x plus optic you will not see the front sight as it will be out of the focal plain, you can't see the front sight. Putting the optic real low also defeats a lot of advantages, so keep it up there anyway.

Good Shootin, Jack

Standard rings would generally be too low.  Like 3rdtk says, the problem is not really clearing the front sight post.  The problem is that with standard rings the scope will be too low and you will not be able to see out of the scope without bending your head almost horizontal.  You should plan on having the center of the scope 1.4" to 1.6" above the rail.  This usually requires a riser, or special rings.

    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's now a similar one available from Compass Lake Engineering.  This gives me an extra 4" of rail and lets 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. I'm using Wiegand Combat rings (available from Brownell's) - they were inexpensive and work well, even though they don't have the macho look many seem to like.