my opinion: It is toxic but far less so than NH3, but for it to reach troublesome levels there is most likely major issues with the tank to allow it to build to that level.I apologize - your reply to my post didn't make sense to me - so - I though that you perhaps mis-spoke. I said exactly what you did in the OP. If there is a x amount of NH4 - there will be a mathematically calculable amount of NH3 based on pH, etc.
My specific question (as others have been discussing) - is whether NH4 itself is toxic.