[wgong]

If you have a spare drive then you only get 1/2 your total storage rather than 3/4. You have data1, data2,parity,spare (although the parity is spread over the three non-spare drives) and it can withstand two drive failures without data loss. If you delete the raid5 and spare then recreate the raid5 using all 4 drives you should get a total of 3x 112 = 336GB (or slightly less). My 705N gets 385GB as I used 160GB drives (which format to 137GB).