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Your question is not very clear. A shed roof does not have a gable.
If I understand your question correctly and you want to use a 4x4 as a beam to support a shed roof for a span of 8' it will sag.
If your overhang is going to be 6' you should not use 2x4s or they will sag also (or fail).
[This message has been edited by GlennG (edited April 22, 2003).]