I also have this underperforming problem with an S4.
Although the "Mining Status" shows 2000+GH/s I only get ~100GH/s @ghash.io and about 200GH/s at eligius.st.
I am using a 10A power cord since we have 220V here (1400W / 220V = ~6,4A) and do not expect that this is the problem.

I would recommend any suggestions on the performance, thanks.