Okay first you have to recognize that the maximum Gravitational potential energy will equal the maximum kinetic energy. The maximum GPE will equal when the stone is at its highest point and the max KE will be right before the stone hits the ground. So
GPE max = KE max
mgh = 1/2mv^2
Mass cancels as it is on both sides
gh = 1/2v^2
Multiply by 2
2gh = v^2
Square root
v = √2gh
Now plug in
v = √2(9.8)(4)
v = 8.85 m/s
Now use the mass to calculate the momentum just before it hits the ground as this is the speed right before it hits the ground
p = mv
p = (3)(8.85)
p = 26.55 kgm/s