The Arduino pins won't be able to sink enough current to make the displays bright enough, given 1:4 multiplex ratio.
Says who?The LEDs probably have a maximum current of 20mA and the Arduino is perfectly capable of driving that. 1:4 isn't a massive ratio.
and I could "modulate" that speed to control the brightness.
I am just wanting to make a "CHEAP & NASTY" clock with 4 LED digits which is plugged into a USB port.I have found the afore mentioned display driver chip but it is a bit expensive.