Well, MAX7219 datasheet says 10mhz clock, that is way faster than 50khz, so I guess, since it supports that, this means it can also display with that speed, right?

Regarding the current project, considering relatively low number of pins needed to drive 4 digits (28 pins) without any multiplexing or whatever, I think, it'll be better, if I control segments directly from MCU. As I have calculated, 1 port high/low or latch high/low with 4mhz clock, is possible at 1mhz frequency. Say I have to update altogether, this means it will be 28 times slower, about 35khz, which is 35000 fps. So if I up clock to 8mhz, I can do 50khz, with some overhead for counter code and so on.