Well....the Si already has PRETTY ADEQUATE anti-sway/roll bars.
I say a good, quality set of coilovers - sleeves or full. Sleeves would be cheaper - but the full-setup (as u know) is much better hehehe
I recall you saying you want the best brands, the BEST parts -so i say go for the full setup.
Tanabe Sustec Pro, A'PEXi WS and N1, Tein, and Zeal are all great setups I've heard great things about.
that'll take abot 1300 outta your 4000 amount.....
leaves enough for i/h/e and a few other things
strut tower bars are good - pretty cheap in terms of most things. DC Sports is good (cheap) but then there's Mugen <img src="http://www.spartanburgracing.com/cgi-bin/ikonboard/iB_html/non-cgi/emoticons/biggrin.gif" border="0" valign="absmiddle" alt='

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peace
boi