Why the serum is 4% niacinamide, not 10%
Somewhere along the way 10% became a selling point rather than a formulation decision.
Niacinamide is one of the better-evidenced ingredients in a cosmetic. It is also one where the marketing arms race has drifted well past what the evidence supports.
The dose-response curve is not a straight line
Most of the published work on niacinamide for sebum regulation, barrier support and tone sits in the low single digits — commonly around 2% to 5%. That is where the effects have been observed most consistently.
Above that range, two things happen. The additional benefit is not well demonstrated, and the reports of flushing, stinging and general intolerance go up. You are increasingly paying for the part of the curve that is flat while taking on the part that is not.
There is nothing dangerous about a higher percentage. It is just that "more" stopped buying much a long way below where the labels are.
Which is why 03 sits at 4%
Four percent is inside the range with the best support, and it leaves headroom to actually use the thing daily without negotiating with your skin about it.
The other reason is that the niacinamide is not working alone. The serum carries 5% of the soluble mineral fraction of multani mitti rather than the clay itself, plus sodium hyaluronate to hold water in the upper layers. A formula is a system. Loading one number up while the rest of it stays the same is not the same as making the product better.
On the pairing myths
You may have read that niacinamide cannot be used with vitamin C, or that it must be buffered against acids. The strong version of that claim comes from conditions — heat, extreme pH — that do not describe what happens on a face at room temperature. In practice most people can use both without incident.
If your skin objects, that is worth respecting. But it is a personal tolerance question rather than a chemistry prohibition, and it is worth knowing which of the two you are actually dealing with.
The honest summary
Niacinamide is thought to help regulate oil production and support barrier function, and the evidence for that at modest percentages is reasonably good. It is not a treatment for a medical condition, it does not work quickly, and a higher number on the bottle is not a proxy for a better formula.
If you are choosing between products, the percentage tells you less than what else is in the bottle and whether you can tolerate using it every day. A 4% you use daily beats a 10% you abandon in a fortnight.