No one decreed that rifling depth HAD to be .004" deep. That depth is/was the norm for US military rifles for many years and rifle makers found they shot well.

Indeed, the most accurate rifles in the world, mostly have .004" depth rifling. It doesn't have to be that deep.

My 9.3x57 has .0065" rifling (per side) (.370" groove depth) and it shoots .366", 293gr. bullets into cloverleafs at 100 meters - 3 shots touching, every time(3 groups in a row), that's 9 shots into less than 1/2" on centres. Of course, being an old worn-out, $275.00, 1929 Husky M94.M96 is probably why.