I will suggest you to let your child play with money but in a way that he learns something from it. Like may be you can sit with him and let him know the value of different coins and tell him which has more value. And later ask him to arrange it as per its value. This way your child's curiosity to play with money is satisfied and he is getting to know about money.

By the age of 4 kids start understanding the relationship between money and shopping. So, best way to let him know the importance of money is involve him in your grocery and other necessities shopping.

You can start from sitting with him and making a list of what all you need to get. You can ask him to tell you what all you all need for breakfast like flakes, milk, some fruits etc. Once the list is done take your son to the supermarket with you and ask him to help you get things. You can tell him what kinds of fruits are available and ask him what he wants. May be he can pick it up and put in the bag. Similarly if you are buying some flakes. Tell him the various flavors and brands that are available and let him know if some particular one is on sale. Tell him since the one on sale will save you some money and its good enough you will get that.

Several such shopping trip will help your kid understand the value of money.

My daughter always kept a new demand for things. How much I tried to tell her that we should not waste money buying things we do not really need, she would not understand.

After getting idea from a friend of mine. I got a piggy bank for my daughter and everyday evening I started giving her a Rs. 1 coin. I asked her to put that coin in the piggy bank. Whenever she asked for something with was for her fun and not important I asked her to see if she has enough money in her piggy. We would sit together and count and if money was not enough I told her the money was not enough. The day she has enough money she bought the thing she wanted.
Now her piggy was empty. She has to start collecting again for her next want

This has helped my daughter understand the importance of money and slowly she is learning to save as well.

I started introducing my son to money as soon as he learned counting. I am trying to explain to him the value of money through day to day activity. Like when I go to buy somethings he wants like chocolate, maggi etc. I take him with me and tell him that I need to give money to get things he wants.

This way I am trying to introduce the concept of need and want also though he is still young but I am sure he will understand by repetition.

Good that your son wants to play with coin.
Actually what I did with my kid in the beginning is I introduce money to her through play. Take different kinds of coins and arrange it according to size. If your kid recognizes numbers you can also tell him what value each coin has. Once your kid knows the value play a shop game. Take all his toys and sell it to him if he gives you a particular coin. This way the kid learns what value money has. Without which he may not get toys.

After you have taught him about coins you can move on to paper money.

Thanks a lot for your ideas. Am trying implement some of your suggestions.

Piggy bank worked for my kids. They love collecting money in their piggy and once its full they put half the money in their bank account and half of it is for them to buy whatever they want. This way they are learning to save and also collect their own money to fulfill their wants.