It doesn't care if you are using simple Volumes or Dynamic Disks in Windows XP. The problem is the partition table. The disk size and partition size is limit to 2 TB. So you can't create more 2 TB partitions on a MBR disk.
The only chance is to make more iscsi targets with a maximum of 2 TB, create a Dynamic Disk on it and make a spanned volume.
From Technet:
Primary partitions and logical drives on basic disks are known as basic volumes. As shown in Table 1.2, basic volumes are limited to 2 TB. Even if you create multiple volumes on a single logical unit, the combined size of all those volumes cannot exceed 2 TB. If you want to use volumes larger than 2 TB, you must use dynamic spanned, striped, or RAID-5 volumes.