The same 4 bit line can actually run upto 2, or 3, 4, 5 or even 6 7-segment display without the need of a mux or decoder or a karnough map. Here is a direct quote from a page:

The simplest way to drive a display is via a display driver. These are available for up to 4 displays. Alternatively displays can be driven by a microcontroller and if more than one display is required, the method of driving them is called "multiplexing."
The main difference between the two methods is the number of "drive lines." A special driver may need only a single "clock" line and the driver chip will access all the segments and increment the display. If a single display is to be driven from a microcontroller, 7 lines will be needed plus one for the decimal point. For each additional display, only one extra line is needed. To produce a 4, 5 or 6 digit display, all the 7-segment displays are connected in parallel. The common line (the common-cathode line) is taken out separately and this line is taken low for a short period of time to turn on the display. Each display is turned on at a rate above 100 times per second, and it will appear that all the displays are turned on at the same time. As each display is turned on, the appropriate information must be delivered to it so that it will give the correct reading. Up to 6 displays can be accessed like this without the brightness of each display being affected. Each display is turned on very hard for one-sixth the time and the POV (persistence of vision) of our eye thinks the display is turned on the whole time. Therefore, the program has to ensure the proper timing, else the unpleasant blinking of display will occur.

FROM: #-Link-Snipped-#

The idea behind is that if you can drive on 7seg, then using the "common line" (common cathode?) between them, something like the /cs of an IC.



Note that your Q is very unclear. What is the 4 bit line? An Uc or a driver circuit? Whats 2 to 12?? Does the 7seg have an internal driver?

SOOraj,Wilsonbat....Check wid this site.this is d best of all ckt. related papers and Designs...May this can be at urs needs #-Link-Snipped-#
Wid Regards,
Just2rock
(Topic and Forum thread replier)