Answer: 3.125x 10^-5 mol/L
In this case, you are given a unit "ppm" or "part per million". Then 4 ppm means 4 part of oxygen for 1 million part of water. Assuming water density is 1000gram/L and we have 1 L solution, then the water weight should be 1000gram. From 1000gram, the amount of oxygen should be 1000gram x 10^-6= 1/1000gram.
After knowing the oxygen mass, convert it into moles. Since oxygen molecular mass is 16, O2 should be 32. The calculation would be: 1/1000gram / (32 gram/mole)= 1/32000mole = 3.125 x 10^-5 mole
The solution is 1L, then oxygen concentration is 3.125x 10^-5 mol/L
