Actually, for the moment, the fourth in line for the presidency is none other than "Sheets" Byrd of West Virginia (President pro tem of the Senate).
From the Law on Succession to the Presidency:
"If by reason of death, resignation, removal from office, inability, or failure to qualify there is neither a president nor vice-president to discharge the powers and duties of the office of President, then the speaker of the House of Representatives shall, upon his resignation as speaker and as representative, act as president.
"If at the time when a speaker is to begin the discharge of the powers and duties of the office of president there is no speaker, or the speaker fails to qualify as acting president, then the president pro tempore of the Senate, upon his resignation as president pro tempore and as senator, shall act as president."
So clearly, it is the President Pro Tempore of the Senate, not the Majority Leader (although they could be the same person), who is fourth in line of succession. You might ask yourself how a Speaker of the House may not "qualify." To be President you need to be at least 35 years old, a natural born citizen, and a resident within the United States for 14 years (Article II, Section 1 of the US Constitution). These criteria are more stringent than those for a member of Congress.