What happens when the words transfered on the bus are smaller than its width?
I would say "it depends":


most likely you ask for 8 bits to be retrieved and you get 8 bits and ignore the other 24 lines; 
if you know you're dealing with sequential data, that you want to consider bytewise, you could retrieve 4 bytes at a time, but you'll have to split or handle the retrieved 4 bytes in code.


On specific architectures you have alignment considerations.

Specifically addressing your question, no, it is very unlikely a system would retrieve 4 words in each cycle only to then "split" them into single words to process, without a program catering for that itself.