HDS may replace the Arc4+. What are Arc4+'s going for on ebay? I hadn't realized they were going for a big premium.
HDS at the moment makes no attempt to replace the Arc AAA, which was Arc's best light and is the one most in demand. It sold new for around $25 and apparently has commanded over $100 on ebay. Similarly, Arc LS series lights have gotten a premium too, though less extreme. But I thought that the premium for Arc4+'s was rather small.
Anyway, the Arc4+ was an Arc/HDS collaboration and so the HDS EDC lights can be thought of as later, improved models of the same light. The Arc4+ was sort of a work in progress when Arc went under.