Need help deciding between two letters of recommendation

[StardustCrusader]

Background: I am applying to another graduate program while in a graduate program of a different institute. My previous mentor recently left and having done 6 rotations here, I found that the lack of funding of most labs I was interested in, as well as the overall environment here is not a good fit for me.

I currently have 4 potential letter of recommendations I want to submit:

1. My previous mentor who recently left (100% will be submitting this)

2. My industry/biotech supervisor I worked with for 1.5 years (most likely submitting this as well)

3. My undergraduate PI (real decision is between #3 and 4 here)

4. One of the rotation professors who I am close with (again, not sure what to choose between #3 and #4).

 

The reason I have a tough time between #'s 3 and 4 is that it may be harder to get a hold of my undergraduate professor/supervisor because he is really busy, but I did not have his LoR on my application to this program last time. However, I also think with #4, I show that it wasn't a fault of my ability to perform/succeed as a graduate student, but it really was the overall environment not being a good fit, which he agreed with.

Edited November 7, 2019 by StardustCrusader