The ZK-4KX (figure 1) is a barebones boost-buck converter module without discernable branding. It is made of vaguely grey plastic with exposed electronics at the back and a black faceplate with blue text at the front. The display is (too) well lit with a bright, monochrome blue LCD. It fits in a 3"x1.5" panel mount hole, with a couple millimeters of flange around the front. There are two buttons, labelled SW and I/O, and one rotary encoder with a silver knob. There are two LEDs labelled CC and ON.
A boost-buck converter, also known as a step-up/step-down converter, is an electronic circuit that can either boost an input voltage to a higher output, or buck an input voltage to a lower output at an efficiency of ~85%. This module is capable of accepting 5-30v DC and outputting 0.5v-30v DC, at up to 4 amps. This does not mean that you can get more total wattage from whatever input source you choose, so choose wisely1.
This is not the most intuitive module in the world, but with the help of a reputable manual you will quickly learn the most important controls. What I find to be most important are the Constant Current feature, the over-voltage limiter, and the over-current limiter. The rest of the features are helpful for semi-automated Li-ion battery charging.
My preference would be to mount this into some kind of enclosure, probably wooden in my case, and hard-wire a permanent input. But, for now, I have bog-standard, pre-wired barrel jacks just hanging off the bare module. This works just fine for my needs. Many, if not most wall-warts will fit these jacks. I've made a pair of alligator-clipped leads that end in a barrel for the output side, along with some other adapters.
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1 The efficiency of the module is rated at 85%. To make the math simpler, and to give ourselves wiggle room, let's count this at 75%. This means that the output wattage must be 25% smaller than then input wattage. For instance, if your input is 12v 1a (very common in junky wall-wart transformers), then your output could be 12v .75a, 6v 1.5a, 3v 3a, 24v .375a, or anywhere along that curve up to 4a. Maybe I'll put a table here.