RGBintensity_blend

I would like an executeable that takes in 4 images (each one is the same size from a screen capture). It will convert each one to a greyscale 8 bit image (0-256, where 0 is no colour and 256 is full colour - note no colour is not white or black but transparent). It will then combine the 4 images, one into red scale, one green, one blue and the last into transparent to black. Ideally this would be with .bmp, .tif or .png files, not with jpg/gifs as the quality must be maintained. If you think you can do this, I will post a powerpoint with an example. I am sure photoshop can do this, but I need a PC and Linux executeable that can do it. thanks Andre