firstly, I'm a newbie
just to answer your question in short form, no, the dimm setup in channel 0 and 2 does not need to match 1 and 4. (or some variation on that in the case of 8 slot motherboards) The first pair can be both a different size, and a different speed, than the second pair. But identical pairs must be used in a dual-channel supporting bank.
In the case that the rated ram speed (the "ram speed". this is just the best frequency and timing that is assured to you on a set) of the pairs do not match, all ram will be run at the best frequency and timing configuration of the slowest pair. sometimes, some ram will not like to run at a slower timing or frequency than it is advertized for, and this, I believe, is what causes the phenomenon of ram not working properly when you add two sets of different speed, in different channels. I believe this is the higher speed ram having trouble when it is run at a slower speed (which should not happen, usually this is an indicator of bad chips) more on this below. Also I am unaware if some BIOS' fail when they do not find a matching profile between the higher and lower speed ram, rather than just configuring the higher speed ram at the lower speed's specs. This could be them taking into account the previous problem about finicky sub-standard ram not wanting to downclock. and see here from wikipedia "If the motherboard has two pairs of differently colored DIMM sockets (the colors indicate which bank they belong to, bank 0 or bank 1), then one can place a matched pair of memory modules in bank 0, but a different-capacity pair of modules in bank 1, as long as they are of the same speed. Using this scheme, a pair of 1 GiB memory modules in bank 0 and a pair of matched 512 MB modules in bank 1 would be acceptable for dual-channel operation."
"Modules rated at different speeds can be run in dual-channel mode, although the motherboard will then run all memory modules at the speed of the slowest module. Some motherboards, however, have compatibility issues with certain brands or models of memory when attempting to use them in dual-channel mode. For this reason, it is generally advised to use identical pairs of memory modules, which is why most memory manufacturers now sell "kits" of matched-pair DIMMs. Several motherboard manufacturers only support configurations where a "matched pair" of modules are used. A matching pair needs to match in:"
As with most things in the PC space, problems arise from cost cutting **** and marketing that is just a tiny bit away from outright lies, rather than with the actual spec stuff itself.
now if you're interested here is some extra information.
based on what I could find when I looked into this before. some of this stuff is to do with increased variation that isn't present anymore, nowadays with there being two memory controllers, largely, and everything pretty much defaulting to either 1600 or 1333.
In the early days of dual-channel, some of the dual-channel setups were ganged, and not truly independent. two 64 bit channel were ganged together, instead of two separate but synchronizing 64 bit paths. this was apparently evident on some AMD chipsets. this is not the case nowadays. there are two fully independent 64 bit channel wired and controlled to operate together as one. when you had the ganged setup, it was known to cause problems when you had mismatched ram pairs in different dual-channel banks. I think some of what it is talked about is a holdover from that era and time.
Also, it should be noted that on Haswell chips, Intel has implemented a new memory accessing scheme whereby you don't even need the capacity of the ram pair to match in one dual-channel bank. Intel calls this "Flex mode". And it basically uses the memory in dual-channel mode up to the limit of the smaller module, and accesses the rest of the larger module in single-channel. the larger must be in the first bank.
I'm not gonna say you should listen to me over these other fine folks that have experience I lack. but this was a subject I have done a lot of research on so I took the opportunity to collect my thoughts once more and in the process share I what I had come up with. no idea if putting say, your 16gb g.skills in one bank, then, your future 8gb [enter name here] in the second bank would cause the system to be unstable if the second pair didn't like the timings or frequency or whatever. or the motherboard wasn't built for this or such. but the spec says it's all good, any problems you might hear of, may not apply to today's cpu-memory relationship and standard of 1600. but I couldn't say. like was suggested, if you don't mind messing with your ram timings, then you could probably get it working, in the case that something screwy went on to prevent this from working as it otherwise has been designed. it is true that all identical is the safest bet with regards to this stuff.
references:http://www.intel.com/content/dam/www/pu ... asheet.pdf
"(section 2.1.3)http://web.archive.org/web/201109290240 ... epaper.pdfhttps://www.overclockers.com/forums/sho ... p?t=622173