Even though you have written that the 4x1GB sticks worked perfectly well
with XP (32bit) by the nature of the limits of that OS not all of that
memory was being addressed anyway, whether that makes a difference or not to
the following I'm not sure...
When you populate a motherboard with four sticks of RAM rather than 2 AFAIK
it can place more strain on your memory controller and to compensate for
this it is often found that lowering your memory timings will help. Even
though each individual memory stick may have a spd rating of (for example)
4-4-4-12 when ran with all 4 memory slots full it can mean that timings of
5-5-5-18 are needed for stability. Even if you bought 2x2GB sticks of memory
they would have slower timings than 2x1GB.
If you are only relying on the spd value to set the memory timings in the
BIOS then the above problem could well apply. Also make sure that you have
set the correct voltage for the memory sticks. Again for example a
particular Gigabyte board defaults to 1.8v and some sticks need 2.1v and so
an increase of 0.3v is needed.
You could use a copy of memtest to ensure that the memory has no fault,
ensuring first that the above is not a problem.
I assume that the memory is approved as being compatible for your system...?