Last I checked, these are the global variables that the UI uses to control whether the extra actionbars are enabled or disabled. Their values are boolean (true for enabled/false or nil for disabled).
Code:
SHOW_MULTI_ACTIONBAR_1 = false
SHOW_MULTI_ACTIONBAR_2 = false
SHOW_MULTI_ACTIONBAR_3 = false
SHOW_MULTI_ACTIONBAR_4 = true