When you overwrite data on your flash disk it doesn't just go to the same location and replace the data like a hard disk, it writes to a new location and saves that location as "valid" for a given block address.
NAND cells are erased when you need to free up space for new writes - but the erase block is large, so much so it's likely that what you're erasing contains data you need to save. You have to move that valid data to a new place and erase the block and this slows the whole operation down.
If you have very little unallocated space on your disk - more and more operations will require this shuffle and this shuffle is less efficient (you can't wait to erase an 'optimal' block because they all contain 90% or more valid data that must be moved). A packed drive is therefore less efficient, but 2 different size drives at the same % fill won't see the same performance loss because the larger one still has more scratch space to work with (most consumer drives set this aside as an SLC 'cache' to enable snappy performance and a more efficient stripe packing, but that cache shrinks as the drive fills.)
Tl;Dr - you don't need to save 800GB of your 4TB drive, but saving 200GB or so will keep you from seeing any performance loss. If you use any consumer SSD in a 24/7 server workload they'll all hit a wall eventually because they're not designed for sustained performance.
When you overwrite data on your flash disk it doesn't just go to the same location and replace the data like a hard disk, it writes to a new location and saves that location as "valid" for a given block address.
NAND cells are erased when you need to free up space for new writes - but the erase block is large, so much so it's likely that what you're erasing contains data you need to save. You have to move that valid data to a new place and erase the block and this slows the whole operation down.
If you have very little unallocated space on your disk - more and more operations will require this shuffle and this shuffle is less efficient (you can't wait to erase an 'optimal' block because they all contain 90% or more valid data that must be moved). A packed drive is therefore less efficient, but 2 different size drives at the same % fill won't see the same performance loss because the larger one still has more scratch space to work with (most consumer drives set this aside as an SLC 'cache' to enable snappy performance and a more efficient stripe packing, but that cache shrinks as the drive fills.)
Tl;Dr - you don't need to save 800GB of your 4TB drive, but saving 200GB or so will keep you from seeing any performance loss. If you use any consumer SSD in a 24/7 server workload they'll all hit a wall eventually because they're not designed for sustained performance.