It is the same plastic that is on the S3. Yes, many manufacturers are using this on their phones but most of them are switching to more premium materials on their high end flagship phones. Samsung seems to be the one stuck in a plastic mold. Also, that nice metal edge that you reference is just metallic looking plastic, yes, more plastic.
I think that Samsung does make a sturdy phone even though it is plastic but I've been waiting for them to come up with more cutting edge form factor and materials and they seem to keep failing in both of those areas. I may or may not be buying the S4 because other high end phones have deal breakers for me. The HTC One has no SD slot and a non-removable battery. I've had an iPhone and it's just not the phone for me (not knocking quality here, just my preference in OS). I've also been looking at the Nokia Lumia 920 series but don't know if I really want to go back to a Windows phone. MS has done a nice job along with Nokia on their high end phones but it still seems like they are a step or two back from Android and IOS. Maybe in a year or two they will be caught up.
The things I like about the S4 are the screen size, the SD slot, the removable battery and the screen quality. I'll have to look at it, hands on, before I decided what I'm going to end up doing.