Formaldehyde is a gas. It will dissolve in water and when the water is saturated it is 37% formaldehyde. Formalin is a water saturated with formaldehyde (37%) with some methanol added to stabilize the formaldehyde. When this saturated solution is diluted 10 fold you get a 3.7% solution. Usually rounded to 4%.
So you see that 10% formalin is really 3.7% formaldehyde.