irule to catch RC4

A couple of points:

1) How do you know your iRule isn't working? In versions 11.6 and 12.0, RC4 isn't available by default, so unless you've modified the cipher string in use with your clientssl profile, you will never match on [SSL::cipher name] being equal to "RC4.*"

2) Your code assumes that all RC4 ciphers begin with the string RC4. This may or may not be true, depending on the version of BIG-IP software you have in use and what the client suggests when connecting.

I would suggest logging the ciphers in use a few times just to verify that and which RC4 ciphers are in use.

3) Why use a switch here, as you have only one matching condition? What you are trying to do could be done easily with an if statement.

when HTTP_REQUEST {
  if { [SSL::cipher name] contains "RC4" } {
    log local0.info "Your log string goes here"
  }
}

I haven't tried out this code, so no guarantee that it works properly.