What is the easiest way to tell if the engine harness I have is a Mefi 3 or Mefi 4? I don't want to plug my Mefi3 in and fry it.

I dont think there is a difference or two different harnesses

a mefi 4 harness is Definitely different then a mefi 3, now a mefi4, 4a and 4b use same harness.

Well Smitty learn something every day, does power and ground go in on the same pins on a 3 vs 4 ?

so if you open both these, take a look at j connector legend, the mefi3 one is on page 7-24 to 7-27 and tells you color of wire in the j connector cavitys, you can compare it to yours
472681_MEFI-4-8.1.pdf
https://marinepowerusa.com/wp-conten....2L-502CID.pdf

Thank You!