(MT4) Does Meta Trader 4 fully compatible with Windows 8 64-bit?

It's not even compatible with Windows 7. Do not install in \program files* - avoid the UAC issues.

From MetaQuotes' MT4 User Guide:

Just because there are some UAC issues for some users doesn't necessarily mean that MT4 isn't compatible with Windows 7.

Compatible means it works out of the box. Install it in \program files* on Win8 it won't even connect to a server to open a demo. The user guide is xxx.

I wasn't suggesting that MT4 is compatible with Win8 (I don't know if it is or isn't since I haven't tried it). However, whether you agree or disagree, MetaQuotes is telling users (and prospective users) in its User Guide that MT4 does operate on Windows 7. Personally, I run MT4 (build 509) on Windows 7 and each of my MT4 installations are located in /Program Files*. Each of those installations were "out of the box" (I turned off UAC a long time ago). I realize that some people can't turn off UAC (or don't have the required permissions to do so) or don't want to exercise that option, so for those individuals, installing MT4 in another folder is better. But, as a I said before, that doesn't mean MT4 is not compatible with Windows 7.

Anyway, MT4 run very well on Windows 7 even with UAC enabled and installed on \Program Files. I don't tried Windows 8 yet.

This is not true.
Please watch your language.

Yes MT4 is fully compatible with Windows 8/32. I don't tried with W8/64 but I guess it's also compatible.

It works fine under UAC on Vista. I installed in ProgramFiles, I run a user level account. UAC actually protects against automatic live updates. UAC asks my permission and requires an administrator password before allowing MT4 (or any other program) to recieve an automatic update.

UAC does what it is supposed to do, it helps to protects the original installation from unauthorized modification or interference.

If you install outside of program files or turn off UAC or run your computer as an administrator all the time you are basically running a very unsecure environment leaving your applications open to interference by reletively simple malicious code, it is like leaving your car parked outside the garage with the doors unlocked and the keys on the dash.

That is not to say if you put it in the garage and lock it all up it can't still be stolen, it just makes it a lot more difficult.

What is the best folder to install MT4 under Windows 8 64 Bit ? where?

Something like C:\MT4 is probably a good choice.