AVA solid-state and hybrid amplifiers produce essentially double their eight ohm ratings into four ohms, up to the limits of the maximum recommended ratings of their built-in speaker fuses. (Don't exceed the maximum recommended ratings of the speaker fuses for your amplifier. Doing so puts your speakers at greater risk in the event of an amplifier fault.)
The Ultravalve vacuum tube amplifier has the same thirty five watt per channel rating for its four, eight, and sixteen ohm speaker taps.