I went from a 4 bulb to a 6 and I would think you would find a 4 not quite enough. Adequate if $ is a concern for sure but near the bottom is quite dim. I too do not have SPS but found even some softies did not seem "happiest" at the bottom under 4 bulb. You can see some comparisons in my pic thread although pictures don't seem to really capture the brightness well.
http://www.canreef.com/vbulletin/showthread.php?t=45688
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45 Gallon Cube with 10 Gallon Sump, Lighting 8 bulb T5, Vertex IN 80 Skimmer
-2 Occelaris Clowns, Engineer Goby, RBTA, Peppermint Shrimp
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