Spawn Random Prefab Continuously?

I wrote a piece of code but I would like it to spawn one out of two prefabs every 4 seconds. NO matter what I try and where I look I just can’t do it. Help would be greatly appreciated. Thanks. (the second variable is redball) `using UnityEngine;
using System.Collections;

public class BallSpawner : MonoBehaviour {
public GameObject purpleball; public float delayTime = 4f;

	IEnumerator Start()
	{
		var obj = Instantiate(purpleball, transform.position, transform.rotation) as  GameObject;
		yield return new WaitForSeconds(delayTime);
		StartCoroutine(Start());
	}
}`

Read this; Unity - Scripting API: Object.Instantiate
And this: Unity - Scripting API: MonoBehaviour.StartCoroutine

I use this to instantiate: Instantiate(BotToSpawn1, new Vector3(375,0,800), Quaternion.identity);
So something like this should work

void Start 
{
StartCoroutine(Spawn1());
}

IEnumerator Spawn1()
{
yield return new WaitForSeconds(4);
//change new Vector3(XXX, XXX, XXX) with wherever you're spawning
Instantiate(YourThingHere, new Vector3(375,0,800), Quaternion.identity);
StartCoroutine(Spawn2());
}

IEnumerator Spawn2()
{
yield return new WaitForSeconds(4);
//change new Vector3(XXX, XXX, XXX) with wherever you're spawning
Instantiate(YourThingHere, new Vector3(375,0,800), Quaternion.identity);
StartCoroutine(Spawn1());
}

Thank you so much! @putin2001