In popular music is very common to outline the 4 chord during a "bridge" section. Technically you are not modulating because you are still within the diatonic key, and at this point the 4 chord is clearly not functioning as an extension of the 1 or 5. To dismiss it seems frivolous, you would be denying everything the Blues stands for.

As for a 1 to 4 progression I can name at least 2 dozen songs off the top of my head that utilize this function (Feeling Allright, Can't Always Get What You Want, etc..). I think the 4 chord definitely has its own unique aural character, In fact I think its one of the easiest and most distinguishable chords to hear within the diatonic system. Why you would want to invent some theory that disguises the relevance of the sub-dominate is beyond my comprehension.

*this sounds like a very interesting debate, unfortunately I have to run a music camp for the next 2 wks and will probably not be able to contribute....have fun guys.