First of all, making a mistake simply makes you human. It's incurable, so don't let it get you down. Only if you did something and you knew better, would I say you might begin teasing the boundaries of stupidity.
Second, if you're following a recipe that arbitrarily tells you to add something after a certain period of time, without qualifying what stage of the ferment you should be looking for, ought to be avoided. Such folks who do something once successfully and figure that it'll work the same for everyone else are setting new meadmakers up for heartbreak. Two different meadmakers, operating in different conditions, using different honeys, potentially different yeasts or nutrients, different water (with varied mineral content), etc., may be in vastly different places after 4 weeks. In the future, I recommend you follow recipes that identify target landmarks during the ferment, such as the 1/3, 1/2, or 2/3 sugar breaks, and tailoring your actions to what your mead is actually doing, not how much time has passed.
On to your present dilemma, I agree, the amount you added is probably appropriate. And although I personally do not ever use chemical stabilizers, from what I understand, sulfites are primarily used to sterilize the must prior to pitching, or after the ferment is 99% complete. During an active ferment, it would take more than you would want to add. Sorbates simply stop yeast (and only yeast) from reproducing. So at the end of a ferment, a one-two punch of sulfites and sorbate will kill off active bacteria and yeast, and the sorbate will prevent any yeast who survive from reproducing.
Hope that helps.
