It's always best with unbuffered memory to only install 2 modules ⇒ the electrical load of 4 modules notably distorts the waveform of the address and data signals and can have a real impact on reliability. Some BIOS programs will reduce the speed when 4 modules are installed; some will switch to single channel mode; etc. to try and improve reliability. I'm convinced a lot of spurious errors and unexplained crashes are simply due to memory “glitches” caused by waveform distortion.
Switching to 2 x 2gb modules will certainly improve the reliability and stability of your memory subsystem; but the change from PC-8500 to PC-6400 will also slow it down a bit. The best alternative would be to get 2 x 2gb PC-8500 modules 
The key to how much load you're putting on the address & data buses is how many actual memory chips are on the modules. Single-sided modules have 8; double-sided modules have 16. So four single-sided modules present the same load as two double-sided modules. But with 1GB (& above) modules you're most likely using all double-sided modules … so I'd definitely limit the load to two modules if you want the most reliable memory subsystem.
… one of these days the desktop motherboard makers will offer motherboards that use buffered RAM modules (like server boards) ⇒ then you can use as much memory as you want (the load on the address & data bus drivers than becomes one load per installed module instead of one load per memory chip).