According to the P400i QuickSpecs page, this controller does support Online RAID Level Migration:
http://h18000.www1.hp.com/products/quickspecs/archives_division/12400_div_v3/12400_div.html
Assuming your four extra drives are not a part of an existing array and your BBWC is active, this should work.
From ACU, select your existing array and select Expand Array. Add the new drives. You should then be able to choose Migrate RAID/Stripe Size after selecting the Logical Volume.
Note that the migration does take some time and during this process write caching will be disabled.
Have fun!
-Bob