for most still running 4.09, you'll find that, by and large, it's a group of guys that started with modding 4.09 and invested a lot of time and effort on that install.
they did not want to let that all go to waste, to just drop it and move on to a new one.
i suill have an old 4.09 install myself, but tbh, i don't really use it anymore, it's basically taking up HD space, but i still hang on to it for, perhaps, sentimental reasons only.
other 409 users will have their own reasons, but this to me is the general feeling.
for a guy like you, who is new to the scene, there really is no reason to go backwards. you are missing nothing, i assure you.
it's the kind of thing only someone who has come a long way with an older version can explain.
there may be a handful of mods that work only for 4.09, but they aren't many, as i said.
by all means, you can install a 4.09 version, i don't want to put you off that, but why complicate your life learning an older method, which doesn't incorporate all the great new features since then... simply not worth it for a newcomer, honestly.
others will likely give more reasons soon, but that's my take.