test <- data.frame(
  Turn = c("Hi. I'm you an' you are me cos",
          "she'd've been so happy cos with all this stuff goin' on",
          "but we're in this together, because y' know things happens",
          "so you can't, cos well, ah because you know why!",
          "not now because it's too late!"), stringsAsFactors = F)
rx <- "^\\s*(?:\\S+\\s+){0,3}(?:cos|because)\\b.*(*SKIP)(*F)|(?:\\S+[\\s,]+){4}\\b(cos|because)\\b"
Turn <- test[grepl(rx, test$Turn, perl=TRUE),]
split <- strsplit(Turn, "\\b(cos|because)\\b")
Index <- sapply(split, function(x) lengths(strsplit(trimws(x[[1]]), "\\s+"))+1)
test <- data.frame(Turn, Index, stringsAsFactors = F)
test