Ignoring everything that's the same (the greatsword itself, the strength of the fighter) the +4 greatsword does 9 extra points of damage on a full power attack.
The +1 +3d6 Fire greatsword does 4 to 19 extra points of damage (average 11.5) without even using power attack.
We're talking higher average damage and hitting more often.
While you could make an argument about damage reduction, look at it this way:
The +4 greatsword will defeat any damage reduction up to a god's, excluding that of certain spells. This means an average of 16 to 26 points of damage (average 21). Power Attack can raise all of these by 5.
Let us assume the creature in question has 20/+2 DR (the most common of the +2 DRs).
The sword itself will do 11-21 points of damage (average 16). Power attack can raise the damage by 5, meaning with full power attack, each successful hit wil get an average of 3 points past the damage reduction. However, damage reduction does not apply to energy such as fire. Meaning that unless you're fighting demons or devils (or anything else with fire immunity or reduction) you're doing 3-18 points of damage, average 10.5, which gets through every hit. Critical hits change things slightly, but not significantly.
In conclusion: if you're facing creatures with +1 damage reduction, or less, and no fire resistance/immunity the +1 greatsword is better. If you're facing against protected opponents, opt for the +4. If you're unsure, go with the +1 and have someone cast magic weapon or bless on it as necessary.
