HOW TO EDIT A FAUCET IN CMYK TO GET FOUR COLOR BLACK

Edit the RGB originals to have neutral R=G=B value faucets (desaturate or black and white command etc).

If the neutral areas have too much CMY and too little K to ensure neutrality with on press colour shifts using the the standard profile, then use a higher GCR profile.

Otherwise if you have to do this edit directly in CMYK, you would select the faucet in the CMYK image and use various colour correction techniques to reduce the CMY and increase the K. The techniques would vary depending on the image and editing requirements, however channel mixer, apply image, color balance and selective color come to mind – and curves for fine tuning. You will need to be careful not to create colour casts and or greatly change the overall luminosity of the image.

I would use colour samplers set in the highlights, midtones and shadows reporting Lab based values, that way when you are at or close to zero A and B values you know the CMYK mix is neutral (you could use RGB readings for the CMYK edits as well to check neutral values).

EDIT: Here is an example of a color balance adjustment layer set to color blend mode, adjusting separate highlight, midtone and shadow values in a CMYK document with Lab colour based color samplers providing edit feedback on the A and B channel values. Although it is best to start with a neutral RGB image, the following shows what is possible even with a heavily colour casted CMYK image. This is just cast removal, manually changing the black generation is even more involved. Profile conversions are much faster/easier and often lead to better quality results (presuming you have a higher GCR profile for the output conditions).

Here I have manually changed the GCR using channel mixer, you can see the before/after CMY vs. K values in the info panel (a profile conversion would be faster and easier than this manual edit). This requires reducing the CMY values and increasing the K values while attempting to maintain luminosity and colour balance:

I don't know why "faucet" is significant here - but anything you do in Photoshop will normally have 4-color blacks, unless you specifically place content in the K channel only. If you create in RGB and convert to CMYK it will always be 4C.

K-only content is normally created in vector applications like InDesign or Illustrator, using swatches so defined - and mainly for text and other graphic elements that will overprint the other inks.

I assume you have the correct CMYK profile from the printer? You normally can't (or shouldn't) convert from one CMYK to another, for a number of reasons.

Do you mean printing a solid using 100% C 100% M 100% Y 100% K?

I think you need to explain a bit more.

Will the final output be a desktop printer or commercial litho printing?