Hi guys, partition magic doesn't support server 2003, as Scleros said Acronis Disk Director Server 10.0 can. But I do not use this software, as it is $699 for server, too expensive to me. You may search partition magic alternative, I used its unlimited edition on my servers, and it works perfect on each one.
I searched the site, it supports 32/64bit server and all kinds of RAID, I think it will help you, too.
To solve the problem, you need to shrink one of the other 3 partitions and get some unallocated space, and then expand the os partition with this space. All operations can be done on the disk map of main windows, very clear and easy.
There is more info
How to Resize Server Partition? Resize Server Partition with Ease