I want to be able to play videos from sites such as Netflix, BBC iplayer, movies4k, putlocker etc. I will be using the raspbmc or openElec distro.
Is this possible? And if it is how do I go about doing it?
I've been looking into this for quite some time, and the short answer is no.
There are if's and but's, and if you don't want to pay for software, the answer is still no, because:
Pipelight can't be installed on ARM devices, and The Raspberry is an ARM device. (Will Pipelight work on a Raspberry Pi?)
Netflix still won't officially support Linux, since they need their DRM. A hack COULD be possible via Chrome and NSS IF and WHEN someone gets around to making an XBMC plugin that works on ARM. This would still be a hack though. (Netflix support is making its way to Linux, finally (updated))
If you don't mind paying for software, and if you don't mind having a Windows computer running whenever you want to watch Netflix, you could try playon.tv
It will stream to any device that supports DLNA (like XBMC)
The software is about $40, and then you'd need the MPEG license for Raspberry, so it can play the stream from PlayOn.
P.S. It won't stream HD video either.
Install pipelight that will allow Linux to run Silverlight which is used to run Netflix and Shockwave Flash used for most other video sites.
I can't vouch for the source but according to this link Netflix will not work on RasPi (or any Linux system - AFAIK both raspbmc and openelec are based on Raspbian, which is in turn based on Debian Linux). Apparently the XBMC plugin only works on Windows.
A much newer tutorial suggests that there is a workaround using third party (commercial) software called PlayOn Server running on (a LAN connected) Windows PC (which your Pi needs to connect to via the network).
As for the other services you mention, my research suggests that iplayer and movie4k plugins will both work on Pi. Not sure about putlocker (I couldn't find any info)...
[update] As per info provided by Hector Prado (in his answer) it appears that it is indeed possible to watch Netflix on Linux (via pipeline). Perhaps a little hacky, but apparently it works! Have a look at this third party XBMC plugin.