Yo Krypton 174: [I like to ad lib.]
Here is how it should work for your 4 tracker if you want a headphone extender/ or you can buy an amplified version.
I have a Peavey 4 plug headphone box. It does not amplify; I use my MD-8 to control the volume.
THE HEADPHONE PLUG ON YOUR RECORDER IS AN OUT -- take a stereo 1/4 inch patch plug, since I'm sure most headphone plugs are stereo; and this patch plug connects to the headphone unit, whatever type you decide to buy; [watch out for gimmick units with too many dials -- they do not sound too cool]
Now you have your connection to the headphone unit; my Peavey has 4 1/4 inch outputs; so, when I record a vocalist in my studio, I use one patch for her, one patch for me.
If I have a guest there, I have a couple of open patches so the "financial backer" can listen to the take. [ha ha] That's the way it is.
Nothing complicated. But once I ordered a nice cosmetic looking patch box; it had some volume dials; it was no good; I sent it back.
The Peavey I use ran about 60 bucks and I am still using it.
See if you can try/or demo/ a unit before you buy. Or make sure where you buy will take back a unit if you don't like it without charging you a fee/etc.
Keep twiddling the dials man,
Green Hornet