How are you sending your LoR. Through interfolio? Well, for one thing, I can tell you that your overall letter quality will not be diluted in the slightest if you use interfolio. Dental application won't know what letters you have on interfolio; you pick and choose which letters you want to be sent. Four is the maximum, so there's no way to lower your overall letter quality even if you wanted to because you cant send in your fifth letter.
If it is not a hassle to get that 5th letter, you might as well get it. But honestly, it isn't that important.