It seems to be a common practice of smartphone makers to release a flagship device with all the bells and whistles it deserves since it is supposed to be on the list of the most powerful devices at the time and a few months later announce some other version of that device leaving the design almost untouched but with internals updated in the case of a device with a larger screen or downgraded a little bit in the case of the "mini" versions of those smartphones. Samsung has been part of this tradition by releasing Galaxy S devices in the first quarter of the year and then a new Galaxy Note in the third quarter while other companies do not follow such timetable.
Last year, LG used a similar design from the G3 in a variety of devices with screens of all sizes and the G3 Screen was sort of a mixed bag between a mini version in the sense that the screen resolution was downgraded to Full HD but also the size of the screen was increased to 5.9 inches, plus they used their own Nuclun processor, but the results weren't great. Now, according to a report from ET News, LG is working on a larger version of the recently announced LG G4 and this time, they intend to use real metal in its construction. LG has used plastic that looks like brushed metal in the LG G3 and in the current flagship the plastic back was given a squarish pattern or there are several colors of leather to customize it.
One reason that explains why they haven't used metallic elements is that the cost of production using this material is obviously higher and since they are currently not selling as many smartphones as companies that use metal like Samsung or Apple, this could end up in some loss. On the other hand, since people like premium materials, this could make them actually sell more devices and they could perhaps use a metallic frame and options including leather in the back. Aside from the cost, the report mentions that the company could make a small amount of the metallic G4 Pro (not the official name) and see how consumers respond in order to raise the odds of it becoming a good seller.