This week my 4 week plant has been sickly and grew...

I strongly advice to invest in a growtent, as temperatures are way too low, even with the heater. This has stunned your grow definitly. Humidity gets this low because of your heater. Its raising the temperature, but doesnt add moisture to the air. When air is warmer, it can contain more moisture, therefore, when you heat up, RH goes down, unless you add moisture to the air. And the low humidity is a problem, it should be above 40, preferably above 50. Therefore, invest in a growtent, it is much easier to manipulate temps and RH in a tent. And the plant is definitly experiencing heavy N toxicity, its darkgreen and curled down tips are clear signs and its looking bad. Flush her good or even consider transplant. Giving the recommended amount on the bottle in this phase is WAY too much. Start with 0,5 or even 0,25. Less is more. Probably your soil had already some nutes in it, hence you dont even have to add nutes the first few weeks, even when the soil is a light mix. I had/have a N excess while only feeding 0,5 of the recommended dose just twice and only 3 liter in total on bigger plants and in a light soil mix (see week 3 of nu diary). I havent add nutes since then and my plants are growing fast, but they are still showing signs of N toxicity. The low humidity makes things worse, as it causes the plant to damp more, therefor taking up more water and thus N, increasing toxic levels in the plant. She's not looking good, I'd invest in a growtent, study some basic guides and probably start over, but maybe you can save her, I have too little experience to make that call. Goodluck!