RAID 5 is not like RAID 1, these HDD are asymmetric, so, hard to say if the data can be recovered in this condition..
by the way, the RAID 5 is not so safe like people say.. we can substitute a bad HDD with no DATA damaged, but if we have 2 HDD damaged the same time, we LOSE ALL OF OUR DATA
It seems to be very difficult to have 2 HDD damaged simultaneously, but actually, the 4 HDD have very same model and same reference and same manufacturing time, the % of damage of those HDD is almost the same!!
The thing happened often like this : the 1st HDD is damaged so you substitute it and rebuild the array, but with up to 3To data, this will be very very long.. and we have a big chance that the 2nd HDD damaged during the rebuilding !!
So, if you want to stock same important data on your NAS, make it in RAID 10, it's mauch more safer than RAID 5..
in fact, with 4 HDD, the worst cas is RAID 5, worse than 4 HDD in RAW..
If you want the data safe and big capacity, make a RAID 6 then.