It is difficult, perhaps impossible, to learn where the generator state resides in the too concise synopsis of RC4 function given here. We expand here.
(define r (fileVal "RC4"))
r is an immutable value that accesses the functionality as follows:
(define s (r seed))
s abstracts (hides) a new mutable RC4 generator state initialized by seed which is an ASCII string, in the sense of a Scheme string. s is a function that takes a scheme symbol as an argument. s returns a generator, sequential invocations of which mutate the generator s to produce and return a random value.
(define bs (s 'nb))
(bs n) yields the next pseudo random integer with 8n bits and s is thereby mutated.
(define byteg (s 'sb))
then (byteg) yields the next 8 bit integer and mutates s.
(define rn ((s 'rbi) n))
Successive invocations of rn produce a new random number 0≤x<n and mutate s.
In each case the mutable state resides in s.

The RC4 crypto plan is to produce the key-stream for seed s as follows: (define t ((fileVal "RC4") 'sb s)). The jth invocation of this value produces the jth character from the stream. (list (t) (t) (t) … ) is then the key-stream. Some advocate discarding the first 1000 stream values.

Here are my development notes for the code.