Yes, but... You should really have had a plan before you began. Nothing complicated; keep it simple - just "Mother is darker. One daughter has an almost black back, but not quite as dark. The other cats are all lighter than Mum." Then you look for the area in Mother that you can make really dark, or even black. Because that area defines "black" to the viewer.
That's an excellent solution. Personally, I'd have used only the 2B - not necessarily expecting it to darken the darks, but to dull and darken the light content. But I think your solution is betterSo to correct this, I sprayed the drawing today with Winsor & Newton Professional Workable Fixative. It would then take the 4B, and then I burnished with 2B. I'm not certain it looks darker in the photo, but it does in real life.
You see? If you'd realised that before you began, you would have allowed for it when drawing Mother. Also, it's sometimes a good idea to print out your initial composition guidelines (probably reduced in size) and then experiment with values. No detail - just block the values in, so you can see how they interact with each other, and how they affect the balance. It all helps you to form an overall picture of what you're trying to achieve.The cat at 2 pm is a Tortoiseshell, but her fur on her back is almost black (but not as black as the black cat)
....and I've no doubt you always will But, at some point, you do have to know when to stop. Otherwise, you risk overworking rather than improving.I keep seeing little things here and there I want to fix.
Finally, your client won't see your drawing; she'll see her cats - her memories overlaid on your drawing. So, I think it's good to go.