There's no such thing as a free piano...
As others have noticed, free pianos show up on craigslist quite literally all the time. Before taking up an offer for a free piano, stop and think about why so many people might be willing to give up their perfectly-working pianos for free.
1) Moving a piano is a huge pain. If you don't do it yourself (of course you would, you're mustachian!), you would have to pay someone to move it.
2) Like anything else, pianos require maintenance and care, which consume both time and money.
3) Pianos require a pretty good amount of space in your home. (This is my main objection to them.)
4) If your kid decides he/she doesn't want to play piano, then all of your efforts expended in 1-3 have been wasted and you will find yourself putting the piano up on craigslist in the "free" section some time in the next five years.
If you want your child to have the opportunity to learn to play piano, look for an electronic keyboard with weighted keys. These used to be called "digital pianos," I'm not sure what they call them these days. They cost a bit of money, but if you buy one right, it will hold its value as long as you take care of it. It will not require much, if any maintenance, It will not take up much space, it can be stored and transported easily, it will likely have MIDI capability so that when your child gets older and becomes a budding little musician, he/she can connect it to a computer and instantly have an audio workstation. Oh yes, and the child and play it with headphones! :D
No, it will not have exactly the same sound and feel as a genuine piano. Trust me on this: your four-year-old simply won't care.