There have been several rumors recently from Samsung employees that at one point the Korean OEM was indeed planning to abandon the plastic frame of its predecessors for a metal, possibly aluminum body. But as the launch date grew close it looked like adding a metal body would have delayed releasing the device so a plastic body was used instead.
The Android community has been going through quite the debate over what materials should and shouldn't be used on high-end devices. Certainly the HTC One makes a strong argument in favor of aluminium or metallic uni-body frames for smart phones. The HTC One is a beautiful device that feels fantastic in the hand. Many times a metallic frame can also be more durable than a plastic one, even if it isn't always more lightweight.
Samsung is a huge machine with a lot of moving parts. Launching a high-profile devices like the Galaxy S4 is a very complex and sometimes imprecise effort. Engineers, designers, manufacturing experts, suppliers, marketing gurus and a million other generally unrelated fields must all come together in perfect harmony in order for a device to actually ship on time without any major hardware defects that could result in unhappy customers. A company like Samsung is launching the S4 on practically every continent within the space of a few weeks, which makes the process even more intensely difficult. I say all this not to apologize for Samsung, or to excuse what appears to be a mistake in scheduling that may have compromised the quality of their flagship smart phone. It is just important that we occasionally take a step back and marvel at the thousands of skilled workers who bring us modern technology.
I am fine with plastic devices. Plastic is great, it is lightweight, generally durable, and it hardly ever scratches. Recently I have discovered that I am in a minority among the Android faithful, and that is OK. One of the great things about loving the Android ecosystem is that you have choices. Those of you who were disappointed by a plastic S4 can lay your weary heads down on your pillows tonight and dream of a Galaxy S4 made of an alloy of mithril and adamantium... but personally I'm fine with plastic.