RAID5 is good in that it provides you one disk worth of redundancy, you can lose one disk and still maintain all your data.
The issue comes when a drive dies and you need to rebuild the array. Upon replacing the dead disk the controller will start to copy over data and parity information from the remaining healthy drives. What this does though is put a tremendous load on the remaining drives, increasing the risk of one of them failing. If another one fails during the rebuild, your data is gone.
RAID5 was perfectly viable when hard drives were <500GB in size because the rebuild would be fairly quick so the chance of losing another disk would be lower. Now we're looking at 3TB, 4TB or even a lot higher the chance of losing your data is pretty likely. Large arrays can literally take days to rebuild while putting the disks under a large load.
In a 4-way RAID I would recommend RAID10 or RAID5. Pick 5 if the data isn't that important OR if you have a reliable backup, pick RAID10 if you value your data more. Another benefit of RAID10 is there are no parity calculations so a RAID controller isn't needed, you can do it in software and read/write speeds will be really good.