How can increase the slab thickness once poured under sized i.e. Required thick is 6" & at site if found 4". How can increase the required thickness..?

i am sure it will require reinforcement bars on existing slab and pore2 inch Concrete

Further confirm from structural design engineers for proper size of bars and grid size. 

It is very difficult to acheive required thickness once poured less. But to a certain extent one can use the shear connector.. by means of additional reinforcement with additional required  concrete..