I just grabbed the new update to the Safari 4 preview, and I have to say it’s very zippy indeed. I ran SunSpider benchmarks on it and FireFox 3.0.1 on my Mac Pro. Safari 4 is on average 75% faster, but has many results that are in the 200 – 500% range.
The integration of the JS debugger is very elegant (Drosera has been rolled in), but I’m still looking forward to the appearance of FireBug for Safari, which someone is doing as a Google SoC project (S4 adds some FireBug hooks too).
All the new CSS funkiness from recent webkit nightlies is there too, and I suspect many an iPhone and widget developer is drooling over them…