Hi anbuonlymevlsi,
As nisshith said, for a 4 bit ALU there is no need of any pipeline mechanism. Here is an example where we can insert a pipeline mechanisms. Have you heard about the ARM's AMBA AHB protocol, in that protocol there is a pipeline mechanism. There is a shared Address bus for read and write operation. So if we are performing a burst read/write then it will more cycles to finish after the address had latched, so once the address latched then we can insert the next address before completing the current read/write operation. Means the insertion of next address in the address bus before completing the current task for that we can insert a pipeline mechanism..