I don't think there's a lot of subjectivity about this one. Traditionally, when one referred to the Big 8, Big 6, Big 5, or Big 4, they were referring to the largest accounting houses (which, of course, used to house very large consulting organisations as well): Deloitte, EY, PWC and KPMG.
Using the term "Big 4" without a modifier outside this context is likely to cause confusion.
Most people would describe Accenture, CapGemini, BearingPoint and Deloitte as the "old consulting arms of the Big 4/Big 5" rather than as the Big 4.
And if you were talking about McKinsey, Bain, BCG and Booz you'd probably reference the "Big 4 Strat Houses"
Clear as mud?