Hello Duhaas, It depends on how much data is on the 3 disk.  When you added the fourth 500gb drive the San has to restripe the data across all the drives which can take a while if the raid group has heavy I/O.  This process can take as long as a few days if there is high I/O.  Dell best practice is to limit I/O as best as possible so that the expansion will complete faster.  Here is a link to the user guide and on page 37 & 38 it explains about disk groups expansion. support.dell.com/.../HOMA00MR.pdf Let us know how it goes.
Working with a customer who has a dell MD3000i storage array.  Had a raid5 diskgroup configured with (3) 500GB disks, we added a new 500GB disk to the diskgroup on Tuesday around 3 pm, and as of 8 am on Wednesday its still in the process of adding the disk.  Is this kind of expansion time normal with a dell storage array like this?  Seems like 17 hours is a bit extreme????
Dell MD3000i Diskgroup Expansion