I'm telling you the following facts..
The value of inductance that resonates with 3u9F at 4kHz is 400uH.
The load value that damps this LC for -6db at the knee happens to be 5 ohms, just saying.
You haven't given us enough information to be any more precise than this, it just isn't that simple.
.......
However, I think it is an excellent guess considering that many 8 ohm tweeters are at approximately 5 ohms and near resistive around where they are typically crossed, and reasonably flat in their response. 390uH is a commonly available value.