The question is whether you can recover from the extra load during the rest period. If you can recover from the extra work, then that's great! You want to do the maximum amount of work that is recoverable.
It is indeed a trial and error process to learn what you can recover from. I think developing that intuition is part of the sport of lifting, and it requires body insight. Also remember that your recovery potential changes based on what's going on in the rest of your life, particularly what you're eating and how you're sleeping.
Now, if you know in advance that you can recover from 4x5 repetitions of a certain weight, then you might consider increasing the weight until 3x5 is the maximum number of sets you can recover from. I think varying the reps, sets, and weight is good for training. I suggest trying to increase the weights first, and going to more reps or sets when that isn't working.
What is recoverable? Simply, will you be ready to lift more weight using the same protocol the next time you're scheduled to do so? E.G. for a Starting Strength routine, you have recovered from workout A if you can do more work during the next A-type workout. Keep in mind that if you lift 200 for 4 sets this time, then you haven't recovered until you can lift >200 for 4 sets next time (even though the program calls for 3 sets: you're tweaking the program when you add more sets, and there is nothing wrong with that, but you should compare apples to apples when you evaluate your recovery).