As a simple variation on any dice technique, allow players who roll a +4 result to roll again. If the result is positive, add it in to the +4 already rolled. If the result is negative or zero, ignore the second roll. This allows a small chance of results up to +8, which can be lifesaving in a dire situation.
Only a pitiless GM would balance this by requiring additional rolls to see how miserably a person can do on a -4 result, however.