Does it work automatically or do you need to turn it on somewhere i haven't seen any option for 4gb tuning

It just sets one flag in the .exe to let Windows know it doesn't assume anything stupid about memory addressing and can use up to 2³² bytes, or 4 gigabytes of memory without failing. Without that flag Windows will refuse further memory allocation if the game's memory footprint would go past 2 GB.

Yes, it will work automatically if you have a 64-bit system or a 32-bit one configured to grant the userspace more than 2 GB (Google "/3gb" if you are running a 32-bit system with more than 2 GB RAM for some reason). The most notable effect is not crashing as easily at high settings or lots of mods.

thanks for the explanation

Now if we could just get 64-bit executable and stop living in the stone age of pc's