Dunno why you think 4 DIMMs would be slower, that's utter BS. Intel memory controllers are (usually) fixed to 2T command rate. This means module amount doesn't matter. At all.
Infact, 4 DIMM configs are usually faster than 2 DIMMs due to higher level interleaving. Upgrading the RAM to so called "high quality" low latency/high speed modules would yield a 2-5% gain...
my reply:
The recommendation against using four memory modules comes from the fact that a lot of people who buy certain CPUs are going to overclock them, and using more than two memory modules makes that harder to do. Some older Athlon 64 and Opteron processors, which have an on-board memory controller, actually ran the memory bus slower if you installed more than a couple of modules. No such issues exist with current Intel chips, but memory modules still use a chunk of the motherboard's regulator capacity, which can start running low when you're overclocking a quad core chip.
This applies particularly strongly to bus-speed overclocking, the only kind you can perform on a non-Extreme-Edition Core 2 Quad. Increasing the processor bus speed increases the speed of things other than the CPU, and also makes perfect timing more important; you often need to bump up CPU and RAM voltage to get higher speeds to work, and that increases power draw too.
Plenty of people are perfectly successfully getting solid overclocks out of quad-core Intel chips while using four memory modules, but if you're buying a new computer, you might as well pay the small price premium to get the same amount of memory in only two modules.