El eterno femenino

We all know that, in Spanish, when you want to transform and adjective in a noun (to make it an abstract concept, let's say), you can use the so called "neutral" article lo:

(1) Lo femenino

However, if we want to modify this new noun with an adjective, we must turn lo into el (the masculine article):

(2) El eterno femenino

An adjective is ungrammatical with lo in these cases:

(3) * Lo eterno femenino

However, this only works if the adjective is pre-nominal; post-nominal adjectives can stay with lo:

(4) Lo femenino eterno

---which means that (3) is grammatical if femenino is the adjective and eterno is the noun.

What could be the reason?