I have UE900s and Primo8. Prefer The primo with spinfit tips. Never heard HEM4. Sound wise UE900s is a bit lackning in crunch in mids for me. More sparkle, more bass quantity. Primo8 is a bit more muted in the treble, lacks sparkle but good definition in upper mids. Compared to UE900s it sounds mid centric. I personally think the Primo8 is more resolving and has a more circular soundstage. The UE900s is wider. For most people I'd say UE900s is a better fit. The build was horrible a few years ago, don't know about now. Primo8 isn't that much better. If you like a slight V-shape and cash to spare I really like Nuforce/MD EDC. These days I don't care as much about absolute fidelity, the convenience and the fun sound signature when travelling by bus has won me over to using The EDC more and more. Though I haven't bought any in ears in the last few years so can't comment on all of the later releases of hybrids.