I am about to institute some sort of allowance with my son (3 1/2) also because he has recently shown an interest in learning about money. I saw a piggy bank at onestepahead.com (they probably have it elsewhere -- it has won some awards) that has four slots for coins -- one is for savings, one for spending, one for charity, and one for investment. Although I don't think we can really explain all of these (esp. investment) this young, still the bank seems like a good idea.
In any case, I have read (and agree) that one gets much better results in terms of kids learning to be motivated to do work (as kids and later as adults) and value money if they have chores linked to an allowance. At 4 it might not be necessary for your child to have spending money, but at some point it will be, and you certainly don't want to face the consequences of having a kid with no concept of how much work went into getting that money. I think as soon as your child is able to understand money and do some simple chores (even if it's not entirely helpful yet), you can start to introduce money concepts and work ethic. However, I also think that there should be some things that you expect your child to do just because he is a part of the household; otherwise you might end up with a kid who thinks he should get something for everything. Here's what I currently expect my son to do (no allowance for it) and what I will continue to expect him to do for no allowance: clean up his toys, put his dirty clothes in the laundry, clean up if he makes a mess, put his milk in the refrigerator and dirty dishes in the sink, and feed the dog. He does all of these things well (he only needs assistance for the dog feeding). Once his piggy bank arrives, I am considering giving him the following things he can do to earn money: vacuum the floor, mop the kitchen floor, sweep, weed the garden, clean the windows, pick up sticks in the yard, rake (when it's fall), wash the car. These all seem like extra things not directly related to taking care of himself or his belongings. Of course, he won't be able to do these things without my close supervision, but the point is not to get help with household chores just yet; it is to give him an idea of the value of money and the value of work.
One thing I have learned from my husband is that people (or at least I) often have lower-than-necessary expectations of children. My kids are always surprising me with what they are capable of doing, and my husband is always pushing them to do things earlier than I would have. My initial reaction is usually, oh he's too young to be expected to follow directions/dress himself/empty the dishwasher/understand XYZ, and then I see that in fact he's not too young. So to parents who think 4 is too young for an allowance, I'd ask why it's too young. Although I haven't actually started an allowance with my son yet, I have no doubt from our previous conversations that he's ready to learn about money (and has already learned some about it), and he's very excited about getting his piggy bank and doing some chores.