When the CPU writes or reads from the memory and stores the value that was read or written into L1 cache. Do the CPU store the whole block( 1 block , leaves the other three ways empty) or the whole set ( 4 blocks)?
Four way set associative does not mean 4 sets.
It means each set is of 4 blocks.
Individual block can be read or written.
Go through this document for understanding the concept of set associative cache.