In short, the S4 molecule will bind to the receptor in the eye, which will cause a yellow tint to be seen in short amounts of time. If you stop taking the S4, the vision side effects will go away in a few days. It's not serious at all.
I have had the vision sides when I get up to 75mg per day. I usually get more "dark spots" vs the yellow tint. Again, nothing harmful but if you drive at night you might want to avoid S4.