The answer is to go for it. Always. Read the following to get the statistical concept, all data is from 2000-2008 regular seasons:
http://www.advancednflstats.com/2009/09/4t...udy-part-3.html
At 4th & 1 on the opponent's 1 yd line, there is a 68% chance of getting the TD, and a 98% chance of getting a FG. The "expected points" of a TD is 6.3 pts, and for a FG is 2.3pts. Further, if either attempt is unsuccessful, the resulting field position for the opponent yields a negative .5 expected points for them which is a positive expected .5 pts for us.
Summing it up for the total expected points from the attempt:
TD -
(.68 * 6.3) + ((1-.68) * .5) = 4.44 expected pts
FG -
(.98 * 2.3) + ((1-.98) * .5) = 2.26 expected pts
Almost worth 2x as much to go for the TD.