If one m³ of that material holds 4,000 kg of it,
then 0.09 m³ holds
(0.09) x (4,000) = 360 kg of it .
The force of gravity acting on 360 kg of anything
on the Earth's surface is
(mass) x (gravity)
= (360 kg) x (9.8 m/s²) = 3,528 Newtons
(about 794 pounds) .
It makes no difference whether those kilograms are in air,
in water, in vodka, in mud, or in jello. If the whole thing is
on or near the Earth's surface, then that's the force on it
due to the mutual gravitational attraction of the kilograms
and the Earth.
At the same time, of course, there is an upward, buoyant force
acting on the kilograms, due to the water they're displacing.
0.09 m³ of material displaces 0.09 m³ of water, whose mass is
pretty close to
(0.09 m³) x (1000 liter/m³) x (1 kg/liter) = 90 kg.
The weight of that water is (90 kg) x (9.8 m/s²) = 882 Newtons.
In water, that 882N acts upward on the material. So if you
put the material on a scale at the bottom of the pool, it would
appear to weigh only
(3,528N - 882N) = 2,646 Newtons
(about 595 pounds) .
That apparent weight is the combination of gravity (down)
and buoyancy (up). The force of gravity on the material is
still 3,528N, but the material seems lighter than that in water.
This scientific explanation reveals the reason why you're able
to carry your big brother around in the pool but not at home.