Here's a link to the program; simply put a 24-bit RGB or 32-bit ARGB bitmap file in the same directory and enter the desired file to convert in the program, and it'll spit out a 32-bit ARGB in IA4 format. I forget if BrawlBox likes ARGB bitmaps, so you might have to convert it to a PNG using GIMP or Photoshop or the like. If you want to convert an IA4 image back to standard ARGB, just run the converter on it again, since all it's doing is swapping the intensity and opacity values.
EDIT:
Source, for those interested. Should compile right away if you put it in an empty Visual Studio project and change the extension to .cpp. Also, as a fun bonus, if you run the converter on a file, then run it again on the converted file, it converts it to greyscale!
EDIT: Aggh, forgot to account for row padding in 24-bit. Should be fixed now.
Also, Project M 3.0's HUD is amazing, though it slightly bothers me that the numbers are still slanted.