Its probably because your motherboard doesnt support Fourway crossfire. if youre running a crap cpu and hardly any ram what so ever, how do you expect your motherboard to be fully capable of having atleast 4 8x pcie lanes?
at that resolution youd need one card.
you say its not for gaming but for mathematical computations... with 2gb of ram and a old beat up AMD cpu how do you ever expect to even come close to giving even ONE card the amount of information it needs to be fully tested??
so basically you have 4-8gb of video ram but 2gb of system ram, how does that equate to good business to you?
if you have that much video ram you should be looking to either atleast equal that or double that in system ram so you know you can provide the correct amount of information and data to ALL of your cards.
basically you have a Ferrari engine on a Unicycle that has a flat tire. how far do you think your gonna make it?
even a sandy bridge i3 would provide those cards with ALOT more bandwidth and information then that amd dual core crap. You should really consider upgrading to a machine that will actually provide all four cards with enough information to utilize them all.
Also for you not answering what your specs are ( no motherboard mentioning ) and not telling us what programs you plan to use, its hard to help you.
From your specs of "dual core amd and 2gb of ram" its safe to say youre trying to use one of your 6950's on a pcie x1 or x4 lane which will not provide it with enough bandwidth to run properly.
saying since you have enough power to run four cards and it should work is just like someone saying they have enough hard drive space so XXX game should work, its moronic. you leave out everything that makes a computer work and focus on one part. kind of like saying "well my car has enough gas, it should work!" '
if you expect any more help you need to give us a detailed specification list of
MOTHERBOARD
CPU
RAM
DESIRED PROGRAMS