not 100% sure here, but i believe the 84 uses the same bulbs as the 87 #882 halogens very expensive like $10.00 each
I retro'd mine to use the 194's, $1.97 for 2, available at any autopart store, walmarts, radio shack etc.
a bit of work to do that (the retro fit), but in the end, the 194's are readily available, and very common
as a side note the 882's being halogen, are very hard on the circuit board, and will/have actually burn(ed) it ( due to the heat), you will see this effect when you first open the cluster. Also the 194's do not require any handeling precautions like the 882's
The 194's do give off slightly less light than the 882's but not enough differance to bother anything.
for me the bottom line was component longevity, so out for the 882's and in for the 194's
I was at autozone last week and they are starting to carry the super bright LEDs to work in many applications. This would be way cool if there was one that would fit the dash on the C4. Not only could you have something that lit up but it would be cool and the life expectancy would just kick butt. But alas I couldn't find anything however I did find 882 light bulbs locally but they were still 8.00 / a bulb.
I thought there are only 4 bulbs in the older C4's? Oh, and for those of you replacing... don't cheap out, replace them all... it will look better, and you won't have to rip the dash apart again when the ones you didn't replace burn out