Well there is nothing wrong with feeling equal, however if you want to maximise your score, you are going to be a lot better off going for 4x 45's and 2x 35's than 6x 40's.
Afterall, the first scenario would give you an Aggregate of 187 whereas the second would only be 168...
How do you know you have Psych in your Top 4? Anything can happen, and it may come down to you hoping that you will be able to have a strong Top 4 if you do happen to stuff up on exams etc
As far as MBBS, I don't think Chem being in your Top 4 is going to drastically improve your chances if anything at all, the set goal of 35 (It is 35 yeah?) is just a prerequisite, it's not going to get you an interview if you had a crappy UMAT
Just go with the flow for this year, if you do well in Psych well then good for you, you'll have 2 out of 4 set, and you can decide half way through which subjects are going to have the best potential at getting a decent score!