"I am working on a project, which use 8051..."

On what basis did you decide to use an 8051?
Normally, one considers the task to be handled, and chooses a solution appropriate to the task & other constraints (eg power, cost).

As abhishek has suggested, the first question you need to ask yourself is whether you need a processor at all; could you just use discrete components? or standard logic? is there a standard IC that already does this? or a simple programmable logic device?

A 4-bit processor would be serious overkill for a task like this - an 8051 is a steam hammer to crack very small nut!