R: how to generate a sequence with a recurring motif that is interspersed with random characters

I'm trying to generate a sequence of certain letters containing a repeating motif that is interspersed with random letters.

For example: ABXXXXXXXABXXXXXXXABXXXXXXX, where X = A, B, C or D, selected at random.

I also need to specify the overall length of the sequence, change the letters that repeat, and how often they do so (e.g., to make BC repeat every 5 characters).

Sadly, I have only been able to get as far as generating the random sequence of defined length, containing select characters:

set.seed(42)
x <- sample(letters[c(1, 2, 3, 4)], size=200, replace = TRUE)