Ok, just to make sure you know that Toshiba produce a nice and low cost
chip to control 4 x 7seg displays, they do decode, multiplex, driver
scan with brightness control in 16 levels, serial interface and more.
Forget about scan rates and software overhead. :) The problem is;
sometimes you just don't have more processor time to do it, and even so,
it will involve transistors or some other driver chips... sometimes
expensive than this Toshiba solution.

For common anode:
http://www.marktechopto.com/tb62709f.htm
http://www.marktechopto.com/tb62709n.htm

They also produce 4 digit x 7 seg in a single package with 12 pins (6+6,
100mil x 600 mil spaced) code MTN 4456R-11A, so save you some pcb work,
perfect for the above chips.
http://www.marktechopto.com/fourdigit.htm

Marktechopto people are very nice and they deserve this free ad.  They
also have several other optoelectronics products, including drivers for
5x7 dot char displays, and the displays.

If you know some other manufacturer similar chip, email me or post here,
I think more people are interested.

Wagner.