It depends on what RAID you're using. Some RAID arrays use multiple drives for redundancy. Therefore, if all four of those drives were on this kind of RAID array, then you would only have 200GB of available storage, as data is duplicated.
In other RAID arrays, speed and performance is the main goal, where data is spit between the four drives, meaning there would be a total of 800GB of available storage.
Finally, you could have a mix between the two if you liked, where 2 drives are paired up in the redundancy RAID, as are the other two. The two pairs are then put up in the performance RAID, allowing for a total of 400GB of available storage.
Long story short, it depends on what kind of RAID you're using, and there are many more RAID configurations than I have mentioned here.
