Interesting that Photoshop doesn't support indexed PNGs with an alpha channel, and Fireworks does.

It seems that patpat's GIM is 8-bit though - GimConv is a bit silly and doesn't seem to try to count colours used in the least.  If you're using the GimConv supplied with Rcomage, you can just supply the "-bpp4" argument to force it as 4-bit.
Interestingly, even as 4-bit, it still generates a palette with 256 colour entries (the weirdness of GimConv I guess) and there doesn't seem to be any easy way to change that.

Anyway, I've attached the 4-bit GIM version, with a shortened palette, which should be able to replace into RCO Editor, or RCOMage (for the latter, don't forget to change the XML to point to the GIM file rather than the PNG file).