Conditionally replace value of entire group in data.table R

Question

I have simple data.table as follows

structure(list(A = c(4L, 4L, 4L, 4L, 4L, 4L, 4L, 2L, 2L, 2L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,