D.B.
Good for you! 4 isn't too young - you just have to gear the charity to her mindset and comprehension level.
Watch the local paper as well as the Boston Globe for charity needs. You can also contact your town's office of Family & Children's Services (whatever they call it) and ask for info on what their client families need or for a referral to other agencies. You could also check with the police department - they may be able to give you info on needs in local agencies. Just don't call 911 - use the business line or walk in. Also, librarians can be a huge source of information.
Your town probably has a food pantry - that's a good place to start as children understand food. It's a great way to introduce the idea of healthy choices too - pantries don't need donations of candy bars or sugary cereal, they need peanut butter and tuna and even fresh produce (check on their schedule and find out the best time to stop in, then shop the day before). It's great to have your child go through the cabinets with you and choose something to donate - not just stuff she doesn't like or that's expired, but things she DOES like. That's true giving. If she can go with you to the pantry (ideally NOT when clients are there - they deserve privacy and confidentiality), she can see the shelves of food. Explain to her that the people who come do not have to pay the way you pay at the grocery store.
I would discourage you from just having her throw money into the Salvation Army buckets - it's your money anyway, and it doesn't give her a sense of where the money goes. She'll think it's to the guy in the Santa hat, and it doesn't give her any picture of what happens. If she has any money herself, she can give it on occasion - I'm just saying it's not enough to really show where charity goes to.
Another thing you can do is check with your senior center or local nursing home - a lot of older people enjoy young children, especially if they live far from their own grandchildren and great-grandchildren. Your daughter could go there and color some window decorations to hang up in the community room or dining area, or make a paper chain with the residents to hang at the reception desk - call and check with the activities director on what would be helpful or appropriate. They may have things around Christmas time, but the long winter can be pretty bare and lonely for people who cannot get out. Paper snowflakes or any other crafty thing she's capable of would be fine, and a lot of the residents are able to work with her to create something jointly.
Sometimes agencies have a "toy pantry" and "clothing pantry" along with their food pantry. I know of an agency in Worcester that does this - it's far for you, but maybe there's something else local to you. I took my son in there and the staff were great at showing him the room of clothing, toys and household items (pots, pans, dish drainers, you name it) that people need when setting up households. In particular, agencies that service the Latino or the African community are often in need of winter items since many of their clients come from warmer climates and are not equipped with winter clothing. Having your daughter sort through her outgrown clothing (only in good condition) and her toys/games/books is a great way to have her develop a concept of sharing. Every time she says "I wish I had...." see if you can get her to suggest something she already has that she would give up. You might see if she can take something in once a month or even every 2 months, and find out if you can just drop in or if you should schedule an appointment. See if you can set up an area in your home (basement, closet?) where things go that are in good condition and ready to be donated.
As your daughter gets older, you can branch out into other forms of charity.
One thing we did with our son was to get him a divided bank - when he received money for anything (allowance, gifts, even earning money walking neighbors' dogs and so on), he divided it. 70% went to him, 20% went into savings, and 10% went to charity. As the charity money accumulated, we worked together to choose what to do with it. Sometimes we bought something to donate, sometimes we gave cash to an agency. He got to choose where to donate, but not whether to donate at all.
I also think it's important that kids get in the habit of donating all year round, not just at Christmas. Food pantries do reasonably well September-December, but they are really lean in the summer months as people are in vacation mode, schools and Scouts aren't in session so no drives are going on, and so on. Also, low-income children in school lunch programs are not in school, and they go hungry. People think about coats and mittens in the winter, but not about clothing in other seasons. Donating old Halloween costumes in September is a great service too. Checking out spring and summer yard sales for inexpensive items of use to needy families is a great idea too - once you get connected with a local agency and know their needs, you can make some excellent informed choices.
We found that, if we made a habit of donating things with our son, it became part of his thinking and it was no longer something we had to ram down his throat.
Good luck!