Q: i ran an apple hardware test and i got the error code 4MEM/4/40000000:Oxb91c3798    can anyone tell me what that means?

This suggests some RAM defect or failure. Did you run the Apple Hardware Test because your Mac had troubles? You may have one or two modules of RAM installed and either of these could have failed.

It might be a chance for you to upgrade to a higher amount of RAM, if buying some new stuff is required.

It says you have either bad RAM or a bad RAM slot. Reseat the memory and rerun the AHT. If you still get the error, and have enough memory to boot the system in each RAM card, pull one out, run the test again until you either find or eliminate the RAM cards. If you have the original memory that came with the system (assuming you've upgraded), put that in, and run AHT again.  You're trying to determine whether it's a RAM card problem or a bad RAM slot.

For the 27" iMac Mid 2010

CPU 3.2 Ghz, the RAM type is

4 - 204-pin PC3-10600 (1333 MHz) DDR3 SO-DIMM

When these were made the maximum amount possible was stated as 16 GB, but now as larger modules exist, it is possible to fit into 32 GB of RAM. I don't think you need to go to 32 but  it is possible if you want to.

This link is to a reputable supplier, who sell good quality RAM. Their home brand OWC is also guaranteed and works fine.

http://eshop.macsales.com/shop/memory/iMac/2010/DDR3_21.5_27

Bring up your Activity Monitor (on your hard drive in applications/utilties) and click on the system memory tab. Look at page ins and page outs. If page outs are 10% to 15% or more of page ins, you can benefit from more memory. A page out is when the system needs to use some of the physical memory that's currently full. It writes some memory to the hard drive to make room for something else, and too much of this can really impact productivity. In a perfect world, page outs would be zero. And if your page outs are zero, just replace the bad memory with the same size, no need to go bigger.