Buying 4x8GB vs 2x16GB ram

Without knowing the specific spec, I'd go for 2x16. Normally better for expansion further down the line.

2 x16 is best

2x16 definitely. Leaves with the option to upgrade in the future.

To fully answer your question it would depend on bank switching and speeds. However the differences would not really be perceivable to an end user, even if they used a stop watch! I'd buy the cheapest option - 2x16GB would make sense if you intended to upgrade, although by the time the chance of upgrade comes round you may find better options... (edited)

The only big performance difference would be if this is a quad channel platform, in which case you'd want four sticks. That's generally only server systems and very high end desktop and workstation setups though.

Beyond that there's a few minor things that might cause the choice to influence memory bandwidth slighty - there were some benchmarks in recent years showing a slight advantage to two ranks per channel for example.

Very few normal PC applications are performance limited by memory bandwidth though, let alone heavily enough that such a difference will be noticeable.