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One of the fun stories of crypto. RC4 was originally a trade secret -- you had to license it from RSA. Then in 1994 it was published by cypherpunks. But we couldn't certain it was the same algorithm, so it was called "alleged RC4" or ARC4.
Because RC4 is a stream cipher, it works by generating a stream of random numbers XORed with the plain-text to create cipher-text. That means it can also be used as a random number generator, which we call 'arc4random()'.
But weaknesses were found in RC4, so we replaced the underlying algorithm in 'arc4random()' with another stream cipher, ChaCha20. So now when you see "ARC4", it might refer to "ChaCha20".
The original 'rand()' function isn't cryptographically secure, meaning attackers can observe a few outputs and guess the entire sequence. So OpenBSD has taken the radical approach of replacing 'rand()' with 'arc4random()' (meaning, ChaCha20).
It's 'radical' because it can break code that depends upon the predictable nature of 'rand()', but it fixes all that code by naive programmers who assumed it was cryptographically secure.
