Crispy plant

26C should not be causing any heat stress nor would a RH of 60-55%, 70cm or 27 inches hang height of lights is normally ok. but really need the lights make and model to confirm that. Given the other plants are fine suggests its something more to do with the individual plant then it is common environmentals like lights, temps or RH, IMO. To me this looks like something has went off in the roots, likely root rot. Maybe from over watering or compact medium or something getting in their from chance. Not all plants grow the same from seeds, some form better root structures. this is likely why the other 3 are fine. Pick up the pot and feel it. I would bet it is heavier then the others if they where all watered at the same time. Can't do much about it now but let it dry out and hope for the best, Some might say give it peroxide to help combat the root rot but it may exacerbate the issues by adding in more liquid while only partly solving the rotting part. Can always add in peroxide next time it needs water and that should only be given when it needs it not as per a schedule. Best to link a diary or start one for these plants, I wonder if they are being overfed or not given enough runoff when being fed.

That's light burn and nutrient burn. The EC is to high and the light is strong. If you have a strong light you need less nutrients (lower EC). If you have a weak light you need more nutrients (higher EC). Drop the EC some and the light should be around 30K lux. Also check the PH of your run off make sure it's not acidic.

If your plant's fan leaves are folding up longways, it's due to heat stress. Source and images: growdiaries.com/journal/15-cannabis-leaf-problems-and-how-to-fix-them

That looks like heat/wind stress to me. There could be more going on though