If your old 4hp outboard is correctly proped it does not surprise me that it out performs the incorrectly proped 6hp outboard.
There is a high thrust prop available from mercury for this outboard par t# 48-812951A1
8 3/8 diameter and 6” pitch with a different blade design than the standard props
I will have to check that out. I do have the 6 pitch prop from tohatsu on the engine. Im not sure if that is the same. Ok the prop I have is 7.9 inches in diameter and 6 pitch. I'm thinking that adding to the diameter at the same pitch will actually decrease the rpm as it will take more force to turn it. However at this point I would be willing to try just about anything as being stuck with a less powerfull engine than I had bites chunks.