so the 4 stands for 4 ribs, and the 990 stands for the length.
try it and see, if it too long get a 4pk950 or 900 if too short go the other way.
i have had funny lengths that i have had to cut 1 rib off the side as i wasnt prepared to wait for the correct one.
i think the subby with underdrive pulleys was running a 4pk instead of the 5pk as i couldnt get the required length.
if i really unsure, get a piece of speaker wire to wrap around the pulleys, mark the required length. take it too repco and measure it with their fan belt measurer and go from there