Sometimes I have the same problem with pulltabs and screwcaps sounding like coins, but it's usually in trashy environments when this occurs. Hunting in 4 kHz helps knock out steel bottlecaps and deep iron, but some deep aluminum targets will sound like coins. Usually what gives it away is erratic VDI readings. Occasionally deep round pulltabs can ID in the 70s, but again the VDI reading is not as steady and confident as a round target. Even aluminum tokens give steady VDI readings, something about the shape of the pulltab or a smashed screwcap seem to skew the VDI readings a little bit.
Running sensitivity higher than 95 has never been beneficial to me, I usually keep it between 80 and 85 at the sites I hunt - you may only lose a half inch or so in depth all other parameters being equal, but some of this "lost depth" can be made up for by using less silencer and tweaking GB settings, among other things. Remember too that while in 4 kHz, you cannot operate in anything but TX=3. There will be some coins, especially long-buried pennies (in my area) that don't give steady VDI readings due to oxidation and other co-located items. Silver coins seem to be less prone to this, likely from being slightly more conductive and less prone to oxidation that copper.
There's also the audio component of a signal that can tell you if it's worth digging or not. Most coins have a "round sound" and this can be demonstrated by scanning a coin, listening to the audio, and then scanning a piece of copper wire and listening for the difference. Coins and other round items will have a characteristic "sweet sound", while oblong or sharp-edged targets will be more abrupt and inconsistent and will have different VDI readings on a 90-degree scan. Expect long-buried coins hidden in trash to have some variation on a 90-degree scan, but usually no more than 10 points of VDI (4 kHz used, expect less variation from higher operating frequencies). Recently I found several Wheat cents among what amounts to rubble which were buried 50-60 years. I dug a lot of co-located trash as well, and I remember one coin IDed at 69 in one direction (4 kHz), and read 63 on a 90-degree sweep. Use the audio component on these types of targets - not only will you dig more good targets, after time you will "just know" what to dig vs moving onto the next signal.
