Do you really mean “on top”? You can’t print process colors on top of other process colors? I’m assuming you mean the shape contained within the solid black background just needs to be a bit lighter.
Picking up on what others have said, 100C, 100M, 100Y, 100K isn’t a good combination. There’s just way too much ink involved. Something like 40, 30, 30, 100 is far better, but as has already been suggested, ask your printer.
As for how much lighter, these kinds of subtle differences in shades of black are difficult to predict. For example, an absorbent uncoated paper will produce significant dot gain, and your 100% black won’t look that much different from an 85% black. Whereas with a hard, coated stock, there would be an easily seen difference.
Given the variables, and assuming you’re printing this offset, I’d use gloss varnish or, possibly, foil for the shape printed onto the black, which you might even have printed on a matte finish stock or with a matte varnish. In other words, it’s safer and better-looking to obtain a distinction between the blacks using gloss and texture rather than using screen tints of process colors. Another way would be to use Pantone ink colors where you would have some assurance what they’ll look like since no halftone screen would be involved.
If you’re sticking with digital for cost reasons, though, your options are limited, and it will mostly be a guessing game as to how it will turn out.