Yes, it is possible and this question has also been answered several times.
You can substitute a letter even if another has already been downloaded. Unless they have printed out your letters, you will not appear to have more than 4.
Actually, once the letter has been downloaded by a program, you can't get it back. So even if not printed, programs will now see five letters.
One letter (whichever you select) will be marked as "inactive" and have a red X next to it.
Does this hurt you? Hard to tell. Personally, it annoys me because I now have more to review, and it suggests that you can't follow directions. Id' think "you should have only sent me three, and saved one slot if you knew another LOR was in the works" or "this guy is scamming the system, and sending 5 letters instead of 4, on purpose".
That being said, would I not interview you for this? Probably not -- meaning that if I was planning on interviewing you with 4 letters, I'd probably interview you with 5 instead.
There;s a limit. Last year I had someone send me 8 letters. I could care less how good they looked on paper -- no interview for that.
Some programs may be more strict, and it's probably field dependent. They might not interview you for adding a 5th letter. Then again, some program might love your new, stronger letter and offer you an interview based upon it. You'll have to decide how much "stronger" this letter is than the others. Life is full of tough choices, and this is one of them.
