Can a 4x8 beam span a 20 foot pergola?

A 4x8 beam may not be adequate to span a 20-foot pergola without significant support. The ability to span that distance depends on several factors, including the type of wood, load requirements, and local building codes. Typically, larger beams or additional support posts would be needed to ensure structural integrity and safety. It's advisable to consult a structural engineer for specific recommendations.