The only reason the iPad 3/4 weigh more is because when the iPad 3 was release, Apple was hoping that IGZO display technology would be widely available. At the time, yields were low and they had to resort to using Samsung's older PLS display technology. The reason a 2nd light bar was added was because the pixels were so condensed that a single light bar would not provide the brightness needed.
For reference:
http://appleinsider.com/articles/11/11/07/apple_leaning_towards_dual_led_light_bars_for_ipad_3s_retina_display.html
Basically the display used in the iPad 3/4 was a Plan B device. The power sucking display is also the reason why the battery had to be larger, hence why the iPad 3/4 are thicker and heavier.
With IGZO now in full production, Apple will be able to provide a Retina display for the iPad Mini in 2013. It is merely a matter of when. The benefits of IGZO include thinner bezels, cheaper cost of production and power savings (using up to 20% power of traditional panels). Since the display uses the greatest amount of battery life, IGZO should allow Apple to produce a Retina quality display while not sacrificing battery life.
For reference:
http://9to5mac.com/2012/09/01/sharps-igzo-displays-on-display-at-ifa-berlin-better-touch-sensitivity-smaller-bezels-galleryvideo/
Devices such as the HTC J Butterfly are already using IGZO technology and are able to pack a 1080P display in a 5" package. This equates to 440 PPI, well beyond any Apple product so far.
For reference:
http://www.slashgear.com/htc-j-butterfly-5-inch-1080p-smartphone-unveiled-in-japan-17252317/
That device is already available in Japan and will probably be announced in the US on November 13th during a HTC/Verizon event.
Basically, the technology already exists to create the iPad Mini Retina. As to what SoC it will be paired with or when it will be released, it will be up to Apple.
My predictions are for a Spring/Summer 2013 release for the iPad Mini Retina with the following prices.
Current iPad Mini: $329
iPad Mini Retina: $399 (Probably with a A5X processor on a shrunken die)
iPad 4: $499
The iPad 2 will be discontinued due to the move to the Lightning connector.