For those not familiar, I firslty include a link to one of Vivienne's immaculate DC-4s and the DC-3. Having flown on these airplanes, I can personally confirm the immaculate presentation of both.
http://www.airliners.net/open.file/249384/L/ (the DC-4 at Louis Trichardt AFB)
http://www.airliners.net/open.file/249749/L/ (the DC-3 at Johannesburg/Rand)
Thinking about passenger certified DC-4s, my first "guess" would have been to wonder whether Phoebus Apollo ever certified their DC-4s for pax service - I know that they usually use them for freighting, but they were talking about using them for passenger charters too, and looking at other photographs clearly ZS-PAI has sprouted cabin windows between when I photographed it at Rand in 1999 and when Nicolai did the same in 2001.
http://www.airliners.net/open.file/144655/L/ (1999 - no cabin windows)
http://www.airliners.net/open.file/172608/L/ (2001 - now with cabin windows)
As Phoebus Apollo are South African however, I suspect that Vivienne may know more about their current activities than I do!
I presume that our friends in Holland have shut down their DC-4 operations, as their airplane is reportedly back in South Africa again???
Beyond that, I can think of no DC-4s regularly carrying passengers. It is just possible that one of the North American operators - Buffalo in Canada and Brooks Fuel in Alaksa come to mind - have a -4 certified for passengers, although I think not and in any case in the normal course of events these two operators only use their Skymasters for freight hauling (and fire bombing in the case of Buffalo). Reviewing the oldprops site, no Fours other than those I mention above appear to be candidates for passenger carrying.
Andy