Boy, will I second Ray's warning! Don't ever go near a table saw with a bare round or already split log. You are asking for DISASTER, as you can't hold it and move it well enough to ensure the saw blade won't bind and then throw the log right up in your face, or elsewhere.
A chain saw is OK if you can find a way to hold the log well while cutting. A mitre saw, or chop saw, is also OK, except you need a way to grip the log VERY tightly. Otherwise you can have the saw blade bind. However since rotation is opposite that of a table saw, the danger is far less. But not inconsequential.
I am lucky enough to have a band saw, which is about the safest you can use. Even with that you have to be careful. If you know any woodworkers, they are likely to have one.
Perhaps the best option for most people would be to cut discs off the log with a chain saw, about 1 1/2 inches thick. Then split these into segments with an ax.
Just watch out.
Paul is lucky having straight-sided 4x4s to cut, as they hold properly against the fence on a mitre saw.
Virgil