var p : Vector4;
p[3] = 5; // the same as p.w = 5

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Vector4 p;
    void Example() {
        p[3] = 5;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public p as Vector4

    def Example():
        p[3] = 5