The Arc 4s are long out of production, back before the original Arc company went kaput and the new one reformed (the new Arc only makes the AAA at the moment). The Arc 4 is mostly a collector's light now. The "replacement" is the HDS. Henry was the brains behind the Arc 4s, but he's got his own thing going now with HDS. The only light of this kind that's better than the U60XRGT is the soon-to-be-rereleased Ultimate 85 (and 85GT), which'll give you significantly longer runtimes than the 60XR/XRGT. More expensive though, and you'll have to wait a while longer.
I'll agree that the Arc 4's body design is better, if only because of the clip. My biggest beef with the HDS is that the clip isn't that great (seems like it was added as an afterthought). I'm still hoping to find an Arc 4 clip to use when I get an U85GT.