Sadly I have yet to hear the JBL K2 S9900's, but I am fairly familiar with the sound of the S9800SE's and believe they should prove a suitably accurate substitute to the S9900's and their capabilities in a comparison to the S4700's.
The S4700's I heard at an exhibition in Sept. this year with Accuphase gear(E460 int. amp + DP510 CD-player), and they sounded sprightful, dynamic, clean, and rather coherent with a midrange that very much appealed to me. This is not to say the bass and highs were lacking(save perhaps ultimate bass extension, but to me that's not really important with main speakers), but simply that the mids stood out clearly and quite present - as it should, to my ears. Some may find this kind of presentation too up-front and/or energetic, but my sonic sensibilities are much less typical "hifi"(or, what it has turned into) and more towards the imprinting of live acoustic music.
If it's any bearing to you, and I believe it should be with regard to the S9900's as well, the S9800SE's to the best of my recollection possesses a better, and more natural density of tone compared to the 4700's, with more intrinsic fill and an even better sense of dynamic contrast. There's more tight meat to the bones with the S9800SE's, to put it differently, and there's more the uhindered feeling of the music expanding freely - both spatially as with regards to micro and macro dynamics. I'm sure this applies to the S9900's as well, perhaps even in a more outspoken fashion. I'd also say bass extension trumps the S4700's, or perhaps it's simply the nature of the bass from the S9800SE's that to me appears more solid and slightly better balanced with the rest of the audio spectrum.
I guess this's a way of saying, that if you could find a pair of used S9800SE's(and do make sure it's the SE's to get the most of them), or better yet (I presume) a used pair of S9900's, I'd go with them instead of a new pair of S4700's. If this is for some reason not possible or doesn't appeal to you I'd say you're very well off with the S4700's.
