The system can be upgraded to 8GB per Sony's own specs/product literature, Intel, and those of every third-party memory supplier. Before you ask the usual questions, I followed the memory upgrade procedure to the letter in the user manual just to be safe even though I have done it hundreds of times before as I work in IT for a living, the memory module type and specs are correct for the system, both modules work perfectly and have passed tests individually when used in either slot, and those memory slots are fully functional with no problems as is every other piece of hardware in the laptop according to diagnostics. If I have just one of the 4GB SODIMMs installed, the laptop boots normally, but with both 4GB modules, it simply idles when turned on with no activity beyond the CPU and GPU fans humming away. No loading of the BIOS, no error messages, no CPU or HDD activity. I can only conclude that there is a defect that requires a BIOS fix.
Upon further testing, I got the laptop to start up normally with 6GB of RAM, which led me to do some more research into the problem. Turns out there is a flaw with the Intel chipsets that were used with the Core2 CPUs. I realized my HP desktop with a slightly older Intel chipset (but also a Core2 chipset) using the same memory controller and i/o controller design had the exact same issue where it could not boot with more than 7GB of memory, which is why they never offered that model with 8GB of RAM instead of the 6GB they shipped it with. They fixed it in a subsequent BIOS update which I had to install before upgrading the memory.