Question: is a 2x6 stronger than a 4x4?
Answer: The single 2x6 is 68% stronger than the 4x4 for vertical loads as you describe. However, I caution you that it may be a good idea to have someone design this beam properly so the job is done right. If you're willing to have a 4" wide beam, why not create a double 2x6 beam, this will be 237% stronger!