A bug does not know if he is on the 11th floor or on the sidewalk. Humans/animals/and air particles can carry an array of bad things. If you walk through grass or take a walk in the woods, immediatley after returning home stay clear of your ladies, and its best to shower off and quaratine your clothes until they are washed.
But, I dont think the brown spots and the chlorosis are caused from pests this time.
Option 1 : Plant is showing signs of a micro nutrient deficieny. Double check all your nutrients and make sure you are providing at least some amount of ALL the nutrient requirments.
Your plants are still very young and it looks like you have them in some sorta organic based growing media, so it should contain a good amount of nutrients already. I realy dont think you need to add any nutrients at this stage of development. Keep in mind that a definciency of one micro nutrient can be caused by an abundance of another micro nutrient or N-P-K as well.
Have you been giving them nutrients recently? If so, How much do you add per gallon? How often do you water them?
Option 2 : You stated, all seedlings are from the same strain, but you have to take into account phenotypic differences. i.e, Just because its the same strain as the other 3, does not gurantee that it will grow the exact same as the rest. There is always a "RUNT"