"I'm to lazy to look but I believe I have a terastation ts400D. Its a 4 drive station."
ask nasnavigator
https://s3.eu-central-1.amazonaws.com/buffalo-downloads/NASNavigator2.zipi think you have
https://www.buffalo-technology.com/de/productpage/terastation-1400/1) "If I format the drive and add it to the array main array(thus I believe rebuild the array?) it seems like that would delete all data on the array?"
The nas can repair a raid with one missing drive. With 2 demaged disks you have lost your data.
2)" I need to back up all my current data"
you should not able to read date from an not running raid
"format that drive, get a new drive and add them in and rebuild the array"
No, that takes a long time. Delete the old array and create a new one.
"3. Stupid question 1, all the drives need to be the same size? If I buy an HP 500gb one and a seagate 500gb one, would there be issues?"
The replacement have to be bigger. If you try 2 differnet 500GB drives, the new one have to be the same or bigger. One missing kb can be a problem.
"4. Stupid question 2, I have read of ways but is there actually a way to put in bigger drives than what is allowed?"
No, your kernel can only handle disks up to 16TB and the whole volume can not be biggen than 16TB. So the optimal soltion is 4x 5TB with Raid5.