I suspect this is a wicked example of Twain's wit and an ironic reference to Canada Bill Jones. The latter was an infamous riverboat and train gambler and thief who famously said "a Smith and Wesson beats four aces". He was also a famous antagonist of preachers and, shall we say, conspicuously pious folk, and a contemporary of Twain.
Canada Bill took to the railroads, and so successfully that once he offered the president of a southern railway company $25,000 a year for the gambling franchise on his trains, allowing that he would not molest the good customers, only those natural enemies of mankind the preachers.
The Courier-Journal from Louisville, Kentucky · Page 46 June 2 1946.
Thus, Bill's typical victims were usually pretty darn happy about their state of affairs, no doubt attributing their seeming good fortune to the just deserts of the righteous, right up until they got had. It's far easier to rob someone after they put all their valuables on the table than before they do.
So I think Twain was actually saying this with a knowing wink to his audience, eg. "I admire the serene assurance of those who have religious faith. It is wonderful to observe the calm confidence of a Christian with four aces - right before they get taken to the cleaners".
Now whether or not this implication was understood by the person who wrote your news article is open to debate. But to me, a "Christian with four aces" can only be someone who is about to receive their comeuppance in a spectacular fashion. And Twain's tone is just dripping with schadenfreude. Note that playing poker is not generally a passtime of the pious. There is quite a lot going on here.
In the news article, the description ' [...] the ethics of a hungry shark married to the confidence of Twain’s “Christian with four aces” ' is equally ironic. And it still reads to me as a naive confidence borne by an unreasonable faith in the idea that the righteous will triumph because they are righteous.