Oh, figured it out myself :)

@layer base {
  .underline {
    text-decoration-thickness: from-font;
  }
}

Yes, you can do this cleanly in Tailwind v4 using the base layer to extend the existing underline behavior:

@layer base {
  .underline {
    text-decoration-thickness: from-font;
  }
}

By placing the override in the base layer, you ensure that any utility-layer class like decoration-4 will still take precedence when you use it in your markup. The cascade works in your favor here: base < components < utilities.

So in practice:

<!-- Gets underline + decoration-thickness: from-font -->
<span class="underline">Default from-font</span>

<!-- Gets underline + decoration-thickness: 4px (overrides from-font) -->
<span class="underline decoration-4">Explicit 4px</span>

This works because underline generates text-decoration-line: underline in the utilities layer, and your base-layer rule adds text-decoration-thickness: from-font to the same selector. When decoration-4 is present, it sets text-decoration-thickness: 4px in the utilities layer, which wins over the base-layer from-font value.

Avoid using @utility for this since you'd be redefining what underline does, which could conflict with Tailwind's own underline utility. The base layer approach is the safe path.