Government in the neoclassic growth model.

Assumptions:

1. g(t)=gov spending per efficient unit of labor=G(t)/[A(t)L(t)]

2. t(t)=lumpsum taxes per efficient unit for labor=T(t)/[AL]

3. government runs a balanced budget t=g

4. government spending (g) has no effect on f'(k(t))

Why assumption 4 is reasonable and necessary?