There are 2.54 cm in 1 inch. Since we are estimating, we can round the average growth per year to .5 cm which would give you roughly 5 years per inch of diameter (2.5 / .5 = 5). Since we have a tree with a 4 inch diameter, we can estimate the age of the tree is...
4 inches x 5 years/inch = 20 years old.