Well, despite what detector companies have or haven't done with their operating frequencies in general, the laws of physics have not changed and there are some fundamental, competing advantages and disadvantages to running low and high operating frequencies on single frequency detectors. Before selectable single frequency detectors and then simultaneous multi frequency detectors started dominating the hobbiest detector landscape, detector manufacturers had to balance the competing properties of low and high operating frequencies. Specifically, low frequency detectors (6 khz or less) give the maximum possible detection depth and couple best with high conductors (or targets with large metal mass) which makes them great silver coin slayers or cache hunters or deep iron hunters (e.g., cannon balls). Higher frequency detector signals (18 khz or higher) are attenuated more by the ground, so they cannot penetrate as deeply as low frequency detectors but they couple better with mid-conductors such as gold, lead, nickels, and brass, so they are more suited to gold prospecting, relic hunting, and micro jewelry hunting. Detector manufacturers limited to a single operating frequency usually ended up splitting the difference at about 7 khz to 13 to 14 khz or they marketed specialized low frequency detectors that favored maximum depth and high conductive/large targets or high frequency detectors (18 khz and higher) to target gold prospecting, relic hunting (where most of the desirable targets are natural mid-conductors comprised of brass or lead), or micro jewelry hunting.
Once multifrequency detectors came along then manufacturers could make detectors like the Deus that could be switched in frequency to accommodate different target objectives or that use simultaneous multi frequency to enable the advantages of both high and low frequencies to be used to detect a wider variety of targets while swinging. Simultaneious multifrequency also enables other advantages such as salt compensation which makes them more stable than single frequency detectors for salt beach detection and also enables better ground balance compensation and advanced features such as iron bias filtering to reduce ferrous falsing.
While true in general, that is not the case with Equinox. I have found, like FloridaSon, that 4 khz actually runs quieter than 5 khz on Equinox with ver 3.0 software, which is counter to what you would expect because another disadvantage of operating lower frequencies is that they are more susceptible to power line EMI. This is telling me that ML may be experimenting with noise suppressing signal processing software and decided to put it out in the field in a low risk manner to see what users think about it so that they can refine it and ultimately utilize it on a future multi frequency detector to perhaps replace Equinox or as a higher end alternative to Equinox. Why did they use 4 khz? Because, no one would really care if it did not work out since 5 khz and the other existing single and multi frequency modes would be unaffected.
If you are swinging an Equinox, I do not see using 4 khz or any single frequency as a primary search mode unless you are forced to because of a noise issue with Multi or if you are really looking for a specific target type that would have an optimal signal at that frequency (e.g., silver at 4 or 5 kihz). If you go to single, for example, you lose iron bias because the iron bias filter is disabled in single frequency - that's why users like Truth may be noticing more falsing when running 4 khz. I really see these single frequency modes as a means to interrogate an iffy signal you discover in multi to try to coax a stronger signal by going to single frequency if the TID indicates the target may be silver, for example rather than being used as a full up search frequency. Just another tool in the toolkit. As long as you educate yourself on the advantages and disadvantages from a high level science perspective, then you might be able to gain an edge in the field, by knowing when that tool might be useful to pull out.
HTH explain better the advantages and disadvantages of the new 4 khz setting on Equinox.
