Basic question about 4-way set associative 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.