I went from the Nitecore D4 (2016 model) to the Nitecore NEW i4, and, after that, to the Nitecore UMS4. The D4, altough it still works, wasn't the greatest choice mainly because removing the batteries from it proved to be tedious when two or more adjacent bays were simultaneously in use, and in fact I even accidentally damaged the top insulator ring on one of my batteries as a result from this. By comparison, the NEW i4 (which also still works) fixed that problem, and, despite it charged the batteries much faster, it ran much cooler, but due to the faster charging it was causing the batteries themselves to generate more heat so the net effect on battery temperature was almost the same as with the D4 so not a significant improvement in that department, with no option to slow down the charging speed other than by making sure at least two bays were kept simultanously charging for most of the time it took to finish the charging on all bays that were in use. This wasn't always very practical TBH, but certainly not a dealbreaker to me back in the day when this was still the best 4-bay charger from Nitecore.
The UMS4 generates even less heat within itself, and, because it lets you manually adjust the charging rate in small steps separately for each bay and also because it also offers faster charging (for those rare occasions on which faster charging pans out necessary...), this is the charger that I generally recommend right now. The fact it uses a micro USB port is not a dealbreaker, as all it takes is just that little bit of extra care to avoid applying any brute force so that it doesn't break. (I hardly ever remove the USB cable from it anyway, as the average user shouldn't need to remove it frequently, when you can just unplug the other end of the cable instead─or better yet, just unplug the USB wall adapter piece from the wall outlet.) Another fine bonus is, the bays are also improved... better than any charger that directly competes with the UMS4. With the UMS4 they are longer so removing 20700 or 21700 batteries is far more practical and they have better quality sliding brackets and better-designed contacts also in addition to this.