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.
Oh yeah!
so the person trying to find the correct diff should stop trying as... (to resolve their diff issues)
"Pool will always assign difficulty equal or greater than this value. It is recommended to leave the value empty (it will be assigned automatically)"
And reading the quick updates (left bar on slush)
14.10.2014
Dear Bitmain Antminer S4 owners, we have had reports about incorrect hashrate being displayed in your profile. After investigating this issue, it turned out that the S4 firmware ignores difficulty lower than 2048. You can now set the minimum difficulty in the worker registration table. This ensures that our stratum servers will NOT attempt to give your workers lower difficulty than this threshold.