CT posted a trip to the past for me. I still believe the OP in that 2012 thread had found some very bad info combined with using poor-quality RAM.
I had a Mid/Late 2007 MacBook Pro 15-inch that ran great on 4GB of Other World Computing RAM until the Dreaded nVidea Curse killed the graphics hardware (unrelated to RAM--design defect instead) and rendered the computer unusable. Then I took the upgrade modules out of my dead MBP and installed them in my mother-in-law's early 2008 17-inch where they have worked perfectly until this summer when, at 95, she decided a computer was getting too hard to use and got an iPad instead. I dragged her MBP out of the closet this week and, although the battery had failed, the computer and its 4GB RAM were still happily working.
The 6GB option is real albeit unofficial but requires a very expensive 4GB module be matched with a regular 2GB module. Based on unit cost of storage the special 4GB module is almost three times the cost per GB as regular RAM. I would recommend leaving your 4GB and avoiding resource hogs like Google Chrome, anti-virus, or any so-called "cleaning/tune-up" apps. The reason: your model contains the same ticking time bomb as mine did--the nVidea GeForce 8600M GT video chipset. When it fails, which it will, the computer is virtually unrepairable because all replacement logic boards (the only fix) have already been used up in repairing the staggering number of computers that failed. When yours fails you are left with expensive RAM that doesn't work in any newer Macs.
Anyway, Apple, MacTracker, EveryMac.com, OWC,Crucial, and most other RAM vendors say 4GB is the official max RAM for your model. OWC is the only vendor of the special module that gets you to 6GB but, given the reasonable life expectancy of your computer and the high cost of that module, I don't see the cost v benefit formula working out for most users. However, if you are like me and like to tinker and can afford it, I will not complain. When we tinker we learn.
PS: anything more than 6GB will cause the computer to fail to boot. Been there, done that.