simple osmosis question. first look at the concentration of NaCl in the H2O. Without even looking into raw numbers, you will have higher concentration of salt in the 4L water and 300g of salt over the 2L water and 100g of salt. If the crappy plants are allowed to equilibrate at 100gNaCl in 2 L H20, putting it in higher concentrated salt water will pull water out of the seaweed cells because the solution is hypertonic to the seaweed cells.
sorry about the rambling, but when the system reaches equilibrium, water will leave the seaweed and into the hypertonic solution.