Synchronizing multithreaded loops in C#

[Salamandras]

Greetings
I would like some help on achieving synchronization between multiple threads running a for loop.
In more detail I would like each thread to be in the same iteration with each other thread, meaning no thread would be in the fifth iteration when another is in the second or in the first...
So the results would go
Thread1 = iteration 1
Thread2 = iteration 1
Thread3 = iteration 1
Thread4 = iteration 1
Thread1 = iteration 2
Thread2 = iteration 2
Thread3 = iteration 2
Thread4 = iteration 2...
And not
Thread1 = iteration 2
Thread2 = iteration 2
Thread3 = iteration 1
Thread4 = iteration 1
Thread1 = iteration 3
Thread2 = iteration 2
Thread3 = iteration 1
Thread4 = iteration 1...

Here is some code I wrote:

public class Alpha
    {
        public int index;
 
        public static int threadsDone = 4; //static so that it can be changed by all threads
        private ManualResetEvent waitHandle;        
 
        public Alpha(int index)
        {
            this.index = index;
            waitHandle = new ManualResetEvent(false);
        }
 
        public void DoWork()
        {
 
            for (int i = 0; i < 4; i++)
            {
                lock(this)
                {
                    Console.WriteLine("Thread " + Thread.CurrentThread.Name + " is in iteration " + i);
                    //if the thread is the final thread then notify all threads to exit wait mode
                    if (threadsDone-- == 1)
                    {
                        threadsDone = 4;
                        waitHandle.Set();//notify all threads to exit wait !!NOT WORKING!! it notifies only the current thread
                    }
                    else
                    {
                        waitHandle.WaitOne();
                    }
                }
            }
 
        }
 
        static void Main(string[] args)
        {
            List<Alpha> alphas = new List<Alpha>();
 
 
            for (int i = 0; i < 4; i++)
            {
                Alpha alpha = new Alpha(i);
                alphas.Add(alpha);
            }
 
            foreach (Alpha alpha in alphas)
            {
                Thread thread = new Thread(new ThreadStart(alpha.DoWork));
                thread.Name = alpha.index.ToString();
                thread.Start();
            }
            Console.Read();
        }
    }
 

The problem is 1) the four threads in the first iteration enter the wait mode successfully but when the last thread cals set none of the threads exit the wait mode. 2) the fourth and last thread which calls set to notify the other threads never enters the wait mode again.

Can anyone please spot an error in the code, or provide code or tutorials which could be helpful to achieving this?

Thanks in advance