I was attempting to save a file at 16 color 4 bit depth using Photoshop 4 on a Windows XP laptop. After creating or modifying the image and attempting to save the file as a bitmap (.bmp) the bitmap options would popup though I was unable to choose 4 bit under the depth option. I figured the image may have to many colors and decided to try to switch the “Image Mode” to Grayscale.
To change the Image Mode select Image from the top navigation menu, highlight Mode (which will be the first item in the drop down menu), and then select Grayscale from the pop out side menu.
After you change the mode to Grayscale you will be able to save your image as 4 bit or 8 bit depth in Photoshop. Select file “Save As” and save the file as “BMP (*.BMP, *.RLE,*.DIB)”. Once you click the Save button more bitmap options will pop up and look like the below image.
If you are saving this as a Windows system file image choose Windows, 4 bit depth, and Compress (RLE).
To be able to save at a higher bit depth you will need to change the image mode.