I teach a 4/5 Loop
I am in the fifth grade part of a 4/5 loop. This was my first time doing it. I think there are pros and cons. I liked that we didn't have to spend the first month of fifth grade getting to know each other and could "hit the ground running." I already knew the students' strengths and weaknesses, and they knew my teaching style and class rules. I also liked that I knew what we had covered last year so when the students didn't remember something and said, "We never covered this," I could remind them, "Oh, yes, we did because I taught it to you!" The hardest part for me is that it took some of the students to understand that my expectations are a bit higher now that they are in fifth grade. There also isn't as much direct communication with parents since you are sort of letting them go a bit. I used to do a weekly newsletter last year, and this year, I have the students write more about what we are doing.
I think that one advantage that you have being asked to do the loop now is that you know what your class is like. Is this a class that you would want to have again, or would you be more than happy for them to move on? Also, I think you need to consider which students would want to loop and which wouldn't if they are given the option.
I know I have rambled a bit. Hopefully, I have helped some. It's a tough decision to make. Good luck!!