Are you recently back in the gym after a hiatus or maybe just newer to training?
Have you increased your calories/carbs recently?
Hows your form at the new weight?
Those things, combined with the S4 would be more conducive to a massive jump in strength like that.
Just added S4 to my stack yesterday. I am really excited to see what I can do with GW/MK2866/LGD AND now S4 as well!