re: fumbles on 4th down.

RULE 3
Ball Declared Dead
ARTICLE 3. A live ball becomes dead and an official shall sound his whistle
or declare it dead:
a. When it goes out of bounds other than a kick that scores a field goal after
touching the uprights or crossbar, when a ball carrier is out of bounds,
or when a ball carrier is so held that his forward progress is stopped.
When in question, the ball is dead (A.R. 4-2-1-II).
b. When any part of the ball carrier’s body, except his hand or foot, touches
the ground or when the ball carrier is tackled or otherwise falls and loses
possession of the ball as he contacts the ground with any part of his
body, except his hand or foot [Exception: The ball remains alive when
an offensive player has simulated a kick or is in position to kick the ball
held for a place kick by a teammate. The ball may be kicked, passed or
advanced by rule] (A.R. 4-1-3-I).
c. When a touchdown, touchback, safety, field goal or successful try
occurs; or when an unsuccessful field-goal attempt that has crossed the
neutral zone and is then untouched by Team B, lands in Team B’s end
zone or out of bounds (A.R. 6-3-9-I).
d. When, during a try, a dead-ball rule applies (Rule 8-3-2-d-5).
e. When a player of the kicking team catches or recovers any free kick or
a scrimmage kick that has crossed the neutral zone (A.R. 4-1-3-II).
f. When a free kick, scrimmage kick or any other loose ball comes to rest
and no player attempts to secure it.
g. When a scrimmage kick beyond the neutral zone or a free kick is caught
or recovered by any player after a valid or invalid fair-catch signal; or
when an invalid fair-catch signal is made after a catch or recovery by
Team B (Rules 2-7-1, 2-7-2 and 2-7-3).
h. When a return kick or scrimmage kick beyond the neutral zone is
made.
i. When a forward pass strikes the ground.
j. When, before a change of team possession on fourth down or a try, a
Team A fumble is caught or recovered by a Team A player other than the
fumbler (Rules 7-2-2-a and -b and 8-3-2-d-5).