I'm not sure you understand what you're talking about and why.
The encryption isn't there for 'safety', as I'm assuming by 'safe' you're talking about 'safe from notices'. Problem is, it's not your ISP sending you those notices, private companies, hired by rightsholders are, and they either send to the ISP, who then forward on to you, or they go to court to get your identity. Either way though, your ISP is NOT looking at what you do.
Now that we've established what it's NOT for, let me explain what the encryption IS for. Many many years ago, ISP's started to traffic shape bittorrent traffic. First they used ports (6881-6889 to start with) to identify, then the traffic headers. So, as a result, protocol encryption came along, to futz that method up. It's explained in detail in BEP 008
So to answer your question "How safe is it to encrypt torrent traffic with RC4?" we have to say 'very safe'. The problem is you're mistaking what it is for something else entirely, something which doesn't exist (and wouldn't work anyway)
Sorry.