slush doesn't allow for manual changing of the diff, is that in the S4's gui???
Check your facts, infact slush NOW allows manual setting of diff.
Or, to be strictly accurate, manual setting of
minimum diff - the pool may then set it higher automatically. BTCguild has done the same.
I have set my S3+s to minimum 256 on both pools (balanced 50/50) and that achieves a higher perceived hash rate on both than does 128.