I run tests on RC4 years ago, run it thru a plain text file full of the same char repeated and then run through RC4, guess what? Oh the password is showing every 256 chars, hence the "weak" key.
I developed a newer version of RC4 called RC64, uses a 64K (65536 or 256 ^ 2) key. The randomisation process is very complex and the algo was only just slightly slower than RC4, which is very fast anyway. A graphical representation of the 64K key visualized pure white-noise when the key was viewed in grey-scale. They need to start using mine me thinks :) Oh, and in a 50MB file full of the same repeated char, the password was not even hinted at and no 4 bytes were the same.