Sen. Ted Cruz (R-TX)
Sen. Rand Paul (R-KY)
Sen. Marco Rubio (R-FL)
Gov. Scott Walker (R-WI)
So who are these four? And which of them will be most successful?
EDIT: Most successful for Republican voters that is.
Bear in mind I've no idea who they are, so fill me in.
As you have already noted, three of them are Republican US senators and one is the Republican governor of Wisconsin. All four were present at the Koch bros. summit to kiss the rings of the Koch bros. and obtain their blessing.
Ted Cruz is a "Tea Party" Republican with views that are too extreme to enable his nomination as a Republican Party presidential candidate. He does not believe pregnant women should have the opportunity to abort an unwanted fetus, opposes gay marriage and gay rights, opposes affordable health insurance for Americans, opposes mandating accomodations for the disabled, opposes net neutrality, and opposes checking the backgrounds of people who want to buy guns. He believes the National Security Agency needs to be more intrusive if it is to be effective in monitoring "terrorism." He believes American economic policy should focus on making rich people richer. He believes America should be providing active military support to Israel on a continuing basis, but he is not certain whom we should attack there.
Rand Paul is a "libertarian" Republican who is the only one of the four with any possibility of obtaining the party nomination for president (but he won't be the nominee). He is a doctor who nevertheless opposes abortion under every circumstance. He opposes gay marriage and gay rights, opposes affordable health insurance, and opposes net neutrality. He opposes any attempt to regulate guns (such as limiting the size of magazines and background checks). Unlike Cruz, he is reluctant to commit American military forces to interventions around the world. He opposes Guantanamo-style indefinite imprisonment and supports privacy rights (reining in the NSA). He says he supports immigration reform, but has opposed every attempt to put it into law. He advocates a Herbert Hoover-style "hands off" approach of the government to intervening in the economy. He opposed saving the the US banking and automobile industries by government intervention during the Great Recession (apparently, he feels depressions/recessions are cyclical norms which should be allowed to resolve "naturally"). He even opposes the Federal Reserve monetary system and attempts by the government to control interest rates and inflation.
Marco Rubio is a young (43 y.o.) Cuban-American who was once imagined by some Republicans to be the solution to that party's disdain by recent immigrant groups. He opposes abortion and gay rights. He opposed the "Violence Against Women Act." He favors increased NSA surveillance of Americans to combat "terrorism." He favors massive, massive tax cuts for rich people. He is an adamant denier of climate change - both that it exists and that humans are causing it. Of the four Republicans mentioned here, his economic policy comes closest to what the Kochs want. However, he is too unsophisticated a politician to go much further than he already has in national politics, and the Kochs (I suspect) know that.
Scott Walker (Republican governor of Wisconsin) is too polarizing a figure to go very far in national politics. He is famous for destroying the public employees' unions in Wisconsin and surviving a recall vote initiated as a response to that. He also survived a challenging re-election campaign. Because of this reslience, he has become a darling of the Republican Party. But, aside from union-busting, he has no real accomplishments to his name. Quite a lot of people cannot stand him, and this dislike is passionate. He opposes abortion in every circumstance, opposes gay rights (which he does not like to talk about), and wants to make rich people richer. His views about international politics are not well articulated. He does not like to talk about international issues, because he makes a fool of himself (by virtue of his ignorance) every time he does.
None of these four are likely to obtain the Republican nomination. Rubio's economic views are the most preferred by the Kochs, but I don't think they will support him because they know he would be hopeless in a national election. Paul would be the strongest candidate against a Democrat in a presidential election, but his laissez-faire economic views are not favored by the Kochs.