That's a good question. While I certainly pay attention to the overall volume too, it hasn't been found to predict the progression of the cancer for prostate cancer. The aggressiveness of the cells (how different they look from non-cancerous cells) is apparently a better predictor. They do however now also quote the % of that grade of cell in the biopsy. So 80% 4 and 20% 3 might be a bit more concern than 60% 4 and 20% 3. You have a 4+4, which means that the aggressiveness of the cancer cells in that sample was graded by your reading pathologist at 4 on a scale of 3 (somewhat abnormal) to 5 (as very abnormal,) and that was more than twice as common as 1,2,3, or 5 graded cells in that sample. This is definitely an intermediate unfavorable result as they say, not based on how far your cancer has progressed, but rather how rapidly it seems likely to progress going forward.
You're right, though, that a good mpMRI provides important input as to how much cancer is already present. 12-20 needle biopsies not so much! A resected prostate after removal provides great guidance as to volume, a little late to provide the help you're looking for.
mpMRI's still vary widely as to both clarity and interpretation, hence the lack of reliance on that alone. I ended up having 2 before my surgery, and the one with the more advanced cancer was completely missed by the first MRI, on less modern equipment with less informed interpretation.