I've never had any problems with fox farms soils. I can never see why people say its to hot to start a seedling in.
First thing you need to do is flush your soil! Next thing you need to do is stop feeding just cal-mag. You need to start feeding with a balanced feeding regimen. Start by feeding some grow. Your problem is a calcium/magnesium over dose. Your plants are wanting some nitrogen. Don't feed at a heavy strength. Start off slow 1tsp or 1.5 and watch how they react. They should slowly start to perk up but the damage is done to those leaves. And as squid said, foilar spray them for a quick fix. 1tsp of grow in a gal. Spray on under side of leaves.
Honestly I would just start over if they are 4 weeks old. Remember it's just a weed you can always start over again an learn from the mistakes made.
I've run

OF,

LW,

HF..

OF is a hot soil. Even FoxFarms, if you call them, will admit it's a hot soil and not to run it unless you dilute it with another product, like Light Warrior for young plants. Some can take it like a champ, some can't. All females are different.
Secondly, I don't understand your advice - you're saying to flush it and stop feeding just cal/mag. Realize, he's only feeding cal/mag with distilled water - he needs
more calmag. You often use full dose cal/mag
every watering with RO or distilled water. It has no micro/macro nutrients in it and

OF does
not have enough cal/mag in it. That is one thing the entire

line (from the liquid nutes to the soil) are rather skimping in. Where I believe the issue is coming in, is the alternated waterings with the spring water. It's more than likely dropping the pH. Flushing will not correct this fast enough without causing damage by drowning the roots.
There's no need at all for him to flush his soil if he's fed no nutrients. There's no way that's cal/mag toxicity as he's using distilled water and feeding
half dose every other watering.
I would not start over, I would foliar feed as it's the best way to stop the deficency fast, then I'd start feeding full dose cal/mag every watering as well as the nutrients for the phase your plant is in. Starting over when you hit a speed bump makes absolutely no sense, how will you learn and correct your problems if you don't take the time to fix the issues that arise?
As well,

OF does not have enough buffer in it and HF has practically none. I'd water with dolomitic lime stone a couple times to try and get it worked into my soil if I were you. Other wise you need to be 100% sure your water going is
ON POINT. as you have no real buffer.
Running 50/50

OF/

HF up top and 100% OF on the bottom woo man, that's some hot stuff.
I've used

products, the entire line.. it's not the most forgiving line of products.
Stick to
one kind of water. Distilled is best as you do not know what's in that spring water, spring water pH can range from 5.0 to 7.5... where distilled water is typically 6.5 to 7.0. Ditch the spring water.
Anyways, need some more information -
How far are the lights from the plants?
Are there fans blowing on the plant?
How often do you water, how do you know when it's time to water?
Do you have the ability to get a new pH meter, a more reliable one? (the funds to do so, some of us do not have those funds) - honestly the soil probes are crap, there's one good soil probe out there that growers here use, the rest of them typically suck.