If i dont install it, what happens?
for example can i join to a 4.0 server or anything

depend of wich server, i think atomix is the onlione out there runing 4.0 that will allow you to join whit 3.44, though sniper servers and jelly havent update to 4.0 yet so i am sure you should have no problem joining those servers, in the otter hand storm 4.0 and noobstories will not allow you to join whitout 4.0.

You can also join the BlackIntel AOW server, though we are running TT, anybody can join

I bother much with some features of 4.0 as not being able to use W3D models that make the game look better, not to have a custom stealth effect and the zoom of the sniper, but it is necessary to block all this cheaters. good thing is that even skin can be used, in addition to the 4.0 is more stable and now my fps go to 60 all the time with the vertical sync on, before going to 42 and has not risen higher.

My recommendation: install it. most of the servers they use, it's just a matter of time that applies to servers that are missing and the anti-cheat.

yea, and besides some w3d files are allowed, at liest in st0rm 4.0 i was able to use my custom commando models for havoc, whit out been kicked, perhaps they started to white list some models/skins.

im try to install 4.0 but keep the old one. is it possible?

install it in separate ren copy folder so that way you will not need to reinstall ren in case anythings goes wrong.