The Sparc units are known to lockup when drive 1 dies, due to it being the base drive. The reason the NAS is having issues resyncing is because of that drive 4 having reallocated sectors. The best idea would be to clone drive 4, then try to add drive 1 (with clone 4 already in the system) in order to sync the drive to the array. Below are instructions to clone your drive.
Simple Hard Drive Recovery
Here is a simple guide to quickly recover a failed drive using dd_rescue.
Here is a quick simple guide how to achieve this with a free Linux Live CD and a PC with two SATA connections.
Using a Knoppix 6.2 Live CD for this guide. Can be found at www.knoppix.net
Using dd_rescue command allows you to copy data from one drive to another block for block. This is especially useful for recovering a failed drive. Often when a drive fails, the drive is still accessible, it has just surpassed the S.M.A.R.T. error threshold. dd_rescue allows you to ignore the bad sectors and continue cloning the bad drive to a new healthy drive.
1) Connect your old drive and new drive to your PC
2) Boot up using your Linux live CD
3) Launch a terminal window.
4) Run fdisk -l to make sure the system sees both of the hard drives.
5) Run hdparm -i /dev/sdx on both of the drives to find which drive is your source drive and which drive is your destination drive
6) Once you know which drive is which you can start the clone process.
dd_rescue /dev/sdx(source disk) /dev/sdx(destination drive)
7) You will see the process start, just keep an eye on it, it might take a few hours for the clone job to finish, depending on the size of the drive.
Once the process is complete, there will be no notification, the transfer will just stop and you will see the terminal prompt again.
If you see a lot of errors or see that there is no more data being shown as succxfer: it means the drive got marked faulty by the kernel. At this point reboot the system and make sure you know which drive is which again, as it is possible they lettering might switch. Run the dd-rescue command again but this time with -r option. This will start the cloning again but this time will start from the back of the drive and will make sure to get the data that has not been cloned yet.
There is no guarantee this will recover your data, but there is a very high chance this will work and its free.