4D 2004 has a feature called adaptive cache. A reference for the use of this feature can be found here:
When using the adaptive cache feature, it is important to keep in mind that 4th Dimension allows a maximum size of 2000 MB for the cache. If you try to enter a larger number for the maximum cache size, 4D will prompt you with an error. The smallest minimum cache size that can be specified is 4 MB.
Also note: even if you specify a cache size of 2000MB, the actual cache allocated might be smaller. The database cache must be a contiguous block of memory and, depending on how fragmented the memory is on your machine, the operating system may not be able to provide the requested memory.
Also remember that 4D can only address up to 2GB of memory (2048 MB). Were you to have a cache that is truly 2000MB in size, only 48MB would be left for 4D.