RAID with 4 drives offers very minimal speed improvement
Solution 1:
If your enclosure is attached to your 2003 server by one single eSATA connection then that's your issue: the maximum throughput of eSATA is the same as a single disk.
If this is the case unfortunately the only real benefit you'll see will probably be seek time improvements.
In order to get a decent speed on the array you'll need an internal controller connected via pci/pcie and either get internal SATA/SAS drives to attach to it or, if you can find one, use a RAID controller that can attach to each disk externally via eSATA.