The E-Trac is a bit different than the Explorers and the Quattro/Safari and many of us find it seems to work better for us and from what we have seen and it may have a slight depth advantage, but 4 inches is stretching it a bit. What I have seen with the one I use is that the deeper signals we can hear them a little better and comparing that signal with the Explorer it too will see it and give a good signal too, but not as nice of a signal. We haven't seen a target the E-Trac get the Explorer will not, but it seems the E-Trac will see it a bit better.
The biggest things I like on the E-Trac is it runs smoother so you can hear the deeper targets better if you don't have your sensitivity to high as auto+3 works great for me while on the Explorer if I used semi auto I lost depth in trashier area. I also like the faster response of the E-Trac and the Quick mask as you can adjust the conductivity or ferrous while swinging the detector with your thumb to check out any target that is iffy. We have also notice it seems to see coins on edge better plus those that have iron close to them as many of the coins we find in the well worked parks and such have iron below or very close to the good target.
We have a couple of guys that hunt together with one has the Explorer XS while the other has the E-Trac and they do find a lot of silver and old coins. One weekend the XS will do better, next weekend the E-Trac seems to find more. The guy with the E-Trac seems to find older and deeper coins with some quite deep, but it seems like these areas are the trashier areas. The guy with the XS went behind the E-Trac one time and picked up 5 merc he had missed, but that can be the E-Trac coil never went over them.
They are close to depth with the E-Trac possibly a slight edge, but not any 3-4 inches we have seen. I feel the E-Trac runs smoother, faster recovery and seems to pick out coins a bit better in trash over the Explorers and Safari which are great detector too, but the Safari to me is a upgraded Quattro like Southwind says.
Rick