Dan.
According to my 2005 handbook for spans, 4x4's 4'o.c. are adequate for a 50lb. snow load for a duration of two months, with the critical value being bending. With proper rafter length, cantilever rules apply (1/3 out 2/3 in). Due to the fact that most snow loads are not evenly distributed throughout the duration of load, great care should be exercised in the design and attachment of kingpins and/or collar ties to distribute the concentrated loads.
Depending on your roof covering and your design two options of reinforcement come to mind: the addition of a fascia member to connect the rafter tails and spread the loads to adjacent rafters, members could be added horizontally from fascia to beam to assist in supporting the large overhang; another option would be the application of tensioned, steel strapping to the top of the rafter, which would supplement the tension chord of the beam reducing bending.
Caution should be taken to ensure the absence of defects near the plate attachment, knots,splits,raising grains, can all reduce the strength of the member, leading to failure.
A good reference for custom applications, just adjust the values for the specie selected.
http://www.spanwood.com
