I guess that would depend on the density of the language - I prefer 2 for clojure / common lisp and 4 for C and python.

At the end, we're trying to optimize for readability, and the horizontal indentation needs to be more or less in harmony with how tight the code is vertically... for example, see this completely random and subjective selection from two of my projects:

    void consumeInput(gap * sudoku, inputQueue * q){
        unsigned short int x, y, i, data;
        input * ptr = q->head;
        q->head = ptr->next;
        x = ptr->x;
        y = ptr->y;
        data = ptr->data;
        free(ptr);
        if(!q->head)
            q->tail=NULL;

        if(DEBUG)
            fprintf(stdout, "%d %d %d:\n", x, y, data);
        x--;y--;



    (defn username-from-req [ring-request]
      (when-let [identity (:cemerick.friend/identity (:session ring-request))]
        (let [[[:authentications auths]
               [:current curr]] (vec identity)]
          (when (and auths curr)
            (str (:identity (auths curr)))))))

(My) C functions tend to be longer but with shorter lines, whereas with clojure, I end up writing denser functions with fewer lines with more meat, both in term of characters and semantics. Thus (again, for me), clojure looks better if lines are more "tight", and C needs more breathing space.

We had people in the same team with the same differing opinions 15 years - we compromised on a coding standard of 3 spaces, and anything else looks wrong to me now.