Quick physics help!

--The buoyant force due to water is 4 (the difference between 30 N in air, and the 26 in water)

--The buoyant force is the mass of displaced fluid.

F(b) = mass_fluid = density * volume

Because of units, the water density is 1000 (units should be kg per cubic meter)

F(b) = 1000 * V(object) = 4

V(object) = 4/1000 = .004 (this seems legit, because it's in cubic meters)

You also know the buoyant force due to the mystery fluid: 6

F(b2) = x * V(object) = 6

where x is your density, and you calculated V in the previous step.

x = 6 / .004 = 1500 (remember, the units are in kg per cubic meter, so this seems to make sense)

As a check, you can look and see that the fluid is 50% more dense than water, and yes, the buoyant force is 50% stronger than in water.