I made royal icing using antonia74's recipe...which I LOVE!, at the end of September. I have stored it in airtight containers. Will it still be good to use to make Halloween cookies next week... (almost 4 weeks later)?

I would play it on the safe side and toss it out
Rather make a new batch of royal icing - this way you know it is right, and fresh, and that no one will get sick or anything from it.

I Allways play it better safe than sorry!!!

Think I would toss. I've had luck freezing Antonia's recipe