var p : Quaternion;
p[3] = 0.5; // the same as p.w = 0.5

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Quaternion p;
    void Example() {
        p[3] = 0.5F;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public p as Quaternion

    def Example():
        p[3] = 0.5F