Okay, here's my $0.02, still not an answer but a few ideas that may or may not make sense in this context.

1) Attach the driver to a two-digit display. Work on it as on standard 4-digit one, and with a single command switch between displays, changing the meaning of the contents i.e. with a pushbutton.
2) Power-saving mode. You can't easily set a digit to half as bright" but you can switch it off for half the time which will be registered as a bit lower contrast. (In the other hand, won't the proces of frequent switching eat up even more power?)
3) As above, lowering voltage. (at school we had a '51 dev board with 6 7-segment LED displays multiplexed. The displays had to be multiplexed in fast sequence because they were powered with 5V and were 3V or so rated. Keeping one lit would quickly burn it, but quickly switching to the next one kept the average voltage lower plus create illusion of all digits lit, not just one.
4) Side effect of savings in board space. Like, this is in fact an internal register that does just that, alternating the contents really fast, but because it "wouldn't fit elsewhere" (i.e. would require fitting another register, which would require increasing the wafer integration scale or making the chips bigger, because it "just doesn't fit in current size",) so they put two internal register bits in place of two unused user-accessible register bits, documented it and allowed (or rather didn't disallow) overriding it by user.


Just my wild guesses though.