The best working set weight would be the max weight you feel you can sustain without getting injured while shooting for 8-12 reps.
Don't worry about grinding out 4 reps. Even Dante Trudel/ Doggcrapp advocates working in the upper rep range (12-15 reps) when you're over 40 years old.
Include rest pause and slower rep speeds to increase the intensity when necessary. Drop weight when necessary to get in another set.
Don't feel you have to get all of your work done in one working set.
I would never shoot for 4-6 reps on squats or really any leg exercise unless you are in a program to prepare for a meet. It's pretty much a universal rule for me to work the higher rep ranges for legs.
If you ever want to try a really challenging program for legs, Google "20 rep squats and RJ Strossen".
There are 3 loves in my life: my wife, my English mastiffs, and my weightlifting....Man, my wife gets really pissed when I get the 3 confused...
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