WEP utilizes the RC4 Stream Cipher from RSA. This is the same cipher that is used in other crypto systems such as Secure Sockets Layer (SSL) (HTTPS). The problem with WEP is that, again, the 802.11 protocol did not define how to implement IVs. As mentioned earlier, the key used in the RC4 cipher is a combination of a shared secret and an IV. The IV is a 24-bit binary number. Many manufacturers will claim to have 64-bit or 128-bit WEP, which is somewhat misleading, since 24 bits of each of these keys are the IV that is sent in clear text. Technically, the shared secret portions of the keys are really only 40 or 104 bits long.
Again, the issue with WEP is not the RC4 Cipher—it's how RC4 is implemented.