Hello everyone.
I have the same problem with the infrared of my note 4, I have never been dropped or hit this is almost new.
But I think I know what is the cause of this problem that I have read in so many forums.
My infrared port worked well with all devices in my house, but after I received an official software update, which was to correct errors in the device and improve the performance of the device. As the 2 days I wanted to turn on my TV With my note 4 did not light I thought it had been deprogrammed and programmed again but I could not turn it on. I downloaded another app thinking that was the problem but I did not work either. Also probre the famous test of # 0 * and I do not know what else and it did not serve.
My conclusion is that the software update deactivates the operation of the infrared port .. !!!
To this day I have not been able to solve that problem I received a further update of performance improvements and software problems but the infrared port still does not work, currently my note 4 has android marshmallow 6.0.1 although when I bought it came with KITKAT But I have been officially updated by Samsung.