Why a 4KB (erase block) flash uses too much RAM
The size of the minimum erase block dictates the size of the RAM buffer required for RMW (read-modify-write) operations, at least in naive implementations. RMW operations are common in filesystem implementations, used for updating data structures such as directories and allocation tables.

It is possible to work around this by writing the updated data to a previously erased block, but keeping track of such relocations can be very tricky. This is usually done as part of a wear-leveling scheme, which is normally a separate layer of software from the filesystem logic.