Application cannot access more than 4 GB on 64 bit processor

Application cannot access more than 4 GB on 64 bit processor

I am trying to run a memory intensive program which requires more than 4 GB memory. I am running it on a 64 bit processor, and I have 16 GB RAM (which is also visible to me). But the program crashes. So I am unable to use more than 4GB for one process on a 64bit processor. How do I solve this problem?