[wonderwrench]

I doubt that there would be anything to worry about at 4 kc. I would be hesitant to drop it much below that, however. But, as with all internet "advice," remember, YMMV.

The Philco "tweeters" were six inch, paper cone, drivers with small voice coils and small field coils so, as you have pointed out, their frequency response would be rather limited anyway. Add the 10 kc filter in the radio itself, and there you go...the radio is not going to reproduce above 10 kc anyway.

Are you speaking of a whistle filter? circuit location?