XDA Forum has a nice post on how to go about partitioning a 4GB (or any size) SD Card for use with your Windows Mobile 5.0 device. This can be used for splitting a 4GB SD card for use with your Windows Mobile device into smaller drives.

Disclaimer: Do this at your own risk. No one can be held responsible if something happens to your SD card.

1) Backup your SD card
2) Install WM5Storage. This software exposes an SD Card as a disk device in Windows Explorer without Activesync installed.
3) Disconnect device from PC (if it’s still connected), launch WM5Storage from My Device\Program Files\WM5Storage\, uncheck “Read-Only” and “Removable Class”, insert the SD Card into your mobile device and Activate WM5Storage (from within the app)
4) Connect the device back to the computer and it should get detected as a new hard drive
5) Go to Computer Management Right click My Computer in Start Menu/click Manage. Go to Storage –> Disk Management where the drive should be listed.
6) Right click on the SD Card drive and choose delete partition
7) Now the SD card will not have any allocated space. Right click again and choose New partition and choose Primary Partition. The size could be anything you want for the partition. For Format type, select FAT32 and perform a quick format. Once done, the partition should show up as Healthy.
8) Now you can create more logical partitions using the same method by right clicking on the SD card drive and selecting New Partition
9) Remember, WM5Storage needs to be deactivated after disconnecting the Windows Mobile device from the PC else your device file explorer will fail to recognize the SD card.

That’s it. Now you have multiple partitions for your SD card which are visible on your Windows Mobile device or for that matter on any mobile device (which supports FAT32) where the card is inserted.