4 Colour Editor

I am looking for a Windows or Linux based editor for manipulating for editing 4 colour pictures.

I then want to save the file to a raw bytes format. The bytes which it saves also need to be packed, so that 1 byte contains the contents of 4 pixels.

I am not interested in a chain of tools, i.e. You draw it in Paint and then import it into somewhere else and then run it through something else.

In this particular instance, I have a 128 x 24 pixels image which I want to be able to write straight to a file of 768 bytes. I do not need any colour information encoded into the file, just pure pixel values.