Q: RAM distribution- Dual processor tray MacPro 4,1

To install memory see:

Mac Pro (Mid 2012 and earlier): How to remove or install memory - Apple Support

The 2009 MP (4.1) came with 1066 MHz memory. Updating the firmware to that of a 5.1 allows running memory at 1333 MHz if 1333 MHz memory is installed.

Sometimes mixing frequently of memory causes problems. I think if you only install 1066 memory it will run at 1066.

When mixing memory note that you can't mix registered RDIMM) and unregistered (UDIMM) sticks

8GB RDIMMs are also supported. The computer will not start if both types of DIMMS are installed in the same computer. For more information, see Mac Pro (Mid 2010): Registered DIMM support.

Mac Pro (Mid 2010): Registered DIMM support - Apple Support

per the link in my first rely memory has to be installed in pairs or as set of three.

This

http://www.everymac.com/systems/apple/mac_pro/faq/mac-pro-mid-2010-westmere-how- to-upgrade-memory.html

says the 2010 will work with either 1066 or 1333 memory but not clear if you can mix them

It is my understanding that the 'optimal' arrangement of memory for the Mac Pro is in 'threes' strange as that might sound although any other permutation i.e. 2s, 4s etc. will work also. Therefore 6x8 or 3x8 + 3x4 might be minutely faster. I found confirmation of this here http://macperformanceguide.com/Mac-Upgrade-MacPro-Memory.html you will see it refers to triple channel memory.

I found the new URL for Apple's official document on how to upgrade the memory in the classic Mac Pro. It is here Mac Pro (Mid 2012 and earlier): How to remove or install memory - Apple Support

As originally determined by OWC it is possible to fit and use more memory than Apple officially say. For a 5,1 dual processor Mac Pro the max is actually 128GB. (As pointed out by Illaass you need to upgrade the firmware for your 4,1 to make it in to a 5,1.)

I vaguely remember that RAM usually needed to be installed in equal pairs.

that rule is for the Mac Pro models from 2006, 2007, and 2008.

The 2009 and later use a different memory organization.

The penalties of less than optimum triples, and the rewards of exactly the optimum triples, are small.

Slots 3 and 4 on each side are "shared", and rarely, you may find combinations of DIMM that are not happy to work when both are populated.

In general, you are free to install one to three DIMMs on each side, and usually four on each side, of mixed sizes, with less than a 5 percent reward or penalty in any configuration.

But 16GB DIMMs are always registered, and cannot be mixed with others. (some older 8GB DIMMs are also registered).

Grant Bennet-Alder wrote:

 

In general, you are free to install one to three DIMMs on each side, and usually four on each side, of mixed sizes, with less than a 5 percent reward or penalty in any configuration.

 

But 16GB DIMMs are always registered, and cannot be mixed with others. (some older 8GB DIMMs are also registered).

I ran into a problem recently when I tried to add an 8GB DIMM to the 4th slot in my 2010 quad core Mac Pro. I had three 8GB DIMMs installed already. But they were all bought in 2012 and the new one was not a match. To get it to work, I was told that I would need to exchange one of the old ones so as to have two pairs of matching DIMMs. So I'm guessing one registered pair and one non?

I ran into a problem with four non-identical 2GB DIMMs, where the exact DIMMs installed in slots 3 and 4 would not work there. I shuffled DIMMs from several different Mac Pro silver towers, and got some that were happy to play nice in all slots.

I don't think they paired up, as another Mac Pro has an 8GB DIMM in the recommended lowest-numbered slot, and three 2GB DIMMs.

I don't think mine could not have been due to Registered/Unregistered because none of them should have been registered -- they were just different makers.

That is why I put so many slippery words in my description above -- three should always work, four will often work.