Hi, I'm new to the forum and found out just a few months ago that my son has moderate hearing loss in both ears. He is almost 4, but the reason we found out so late is that he's lived overseas since birth. My son is exposed to several languages - Russian (primary), English, and Spanish. Since we discovered his hearing loss, we've really been focusing on English to get him ready for school (he will attend the American School in the fall). He now has the option now of attending half-day preschool with instruction in ASL and Spanish. I checked, and they do use ASL (I even recognized some of the ASL signs from books I've been reading). Should he learn ASL, or do you think it will be too hard for him, or confuse him, to learn four languages at once?
Seriously, you are providing him with such great opportunities! How's he doing with the first 3 languages? I'd say expose him to everything possible, watch carefully, stretch him a bit, and go forward with what you think enjoys. I've read so many studies about the benefits of multiple languages, the positive impact on literacy.
I think with Russian, English, Spanish, and now ASL, this child is going to be surviving just fine.