Question

In X and Y do you find new Pokemon on the Routes after you beat the E4?

Answer 1

No the Pokemon remain the same, but you can't get certain items until you have defeated the Elite 4 such as the fossils in Glittering Cave.

Some areas in the game also cannot be unlocked until you have defeated the Elite 4, notable examples include:

The Friend Safari
Unknown Dungeon
Terminus Cave (Zygarde's Chamber)
Sea Sprit's Den

Hope I helped :)

Comments

I got an Old Amber before beating the Elite four (I remember it 'cause it was my Mega Aerodactyl).

Answer 2

The answer is no. The pokémon stay the same.