Comet Linear 1999 S4 can now be seen in the early evening in the light polluted suburbs. I spotted it with my 9 inch SCT last night from south Palo Alto where the northern horizon is far from dark. I could not spot it in my 7x50 binos or 8x50 finder, probably because the tail is very hard to see in these conditions and the comet would be hard to distinguish from a star. I located it by interpolating from the July 20 and 21 positions found in the July S&T (10:00 pm in California on July 19 is after 0:00 UT on July 20) and used digital setting circles to find the general area. Then a sweep using a focal reducer and low power eyepiece and patience was rewarded. I recommend using a wide field telescope if you've got one. The tail was visible at 90x and both broad band and narrow band filters helped define it. The broad band was better.
The comet will be several degrees higher above the horizon every day until early August so now is the time to go for it. If I could see it at 15 or 20 degrees above the horizon between a tree and a basketball backboard so can you, wherever you may be.
One last note. I would be surprised if this is a naked eye object anywhere but the Canadian Rockies.