The following are some important factors to take into consideration regarding this passage:
- The context is speaking on the issue of pride and other sinful attitudes plaguing the Corinthians, and not focused on laying out a rule of faith. The problem Paul is talking about is not one regarding "scripture alone" versus "scripture plus."
- 1 Corinthians 11:2 explicitly tells them to hold onto the traditions Paul handed onto them, thus 4:6 could not be teaching SS.
- Given that various NT Scriptures (e.g. 2nd Corinthians) weren't written yet means practicing SS was (functionally) impossible at that point in time.
- When one looks at how Protestants have historically read 1 Corinthians 4:6, it was not really appealed to as a SS proof text. For example, here is John Calvin's Corinthians Commentary on this verse:
"The clause above what is written may be explained in two ways — either as referring to Paul’s writings, or to the proofs from Scripture which he has brought forward. As this, however, is a matter of small moment, my readers may be left at liberty to take whichever they may prefer."
Those are hardly the comments we would expect for a SS proof text - Calvin says the meaning of the phrase isn't that big of a deal! Taking the first option, Paul was speaking of heeding his warnings in his own epistles. Taking Calvin's second option, Paul simply was telling them to heed the warnings of the OT citations in the previous 3 chapters. Neither option comes anywhere close to suggesting a SS proof.
- The term "written" can mean a whole range of things: (a) the whole Bible, (b) the whole OT, (c) the whole NT, (d) the whole OT plus partially completed NT, (e) Paul's Epistles, (f) Paul's lesson on humility given in 1 Corinthians Chapters 1-3, (g) the OT passages Paul quotes in his lesson on humility, (h) something else.
Evaluating these options, (a) is the only option that satisfies SS, but it is not the only option or even most likely, given options like (b) and (e)-(h) are far more exegetically fitting.
Really, taking these factors into consideration, for a Protestant to suggest 1 Corinthians 4:6 is a proof text for Sola Scriptura is engaging in a blatant fallacy of begging the question. Simply examining the context, the point of the verse is Paul is saying he inserted his name and Appolos' into the lesson on humility he gave in chapters 1-3 so as not to point any fingers at the true culprits and to curb the Corinthian pride (4:6a). For the Corinthians to exalt themselves beyond what Paul wrote in chapters 1-3, especially the OT texts against pride and worldly wisdom (4:6b), is to act outside of the acceptable boundaries of Christian humility (4:6c).
End Note: Much of this was taken from a conversation I had with a Protestant in another comment box on my blog (see comments by Nathanael Taylor, starting March 10).