I have a 4 ton unit that is dead I have friends in A/C they have a 3 ton or a 5 ton that they can give me for free they are used but they work I just don't know which one I need to use.

Too small or too big and you will not be happy. My recommendation would be to get a free estimate from a local and reputable HVAC company to tell you what size you need.

The size that you need will be based on the size and type of home that you have, along with what returns and vents that you have.

If one of the free ones will work, then you'll need to get someone to put it in correctly, so take that into account too.

You can use the 3 ton if you want, inless your indoor coil is a 5 ton coil you an not use the 5 ton outdoor unit.

Josh, do we know enough (size of home, etc.) to be sure that a 3-ton will work for him?

One thing for sure, to big is a bad mistake,to small will run all the time and not do the job as well but better than being to big,

The home should have a schedule J calculation done to determine
What you do need
A start would be for you to figure the square footage of the home