Looks to me like the leaves tword the bottom were sprayed with strong nutes, or possibly got splashed on while watering, and got burned as a result.
Kinda looks like overwatering a bit, too. Have you been watering a bit extra to help remove the nute build-up? Constantly wet soil can cause root rot. Let 'em dry out a bit between waterings.
If it was too strong of a nute mix, every time you water the plant with plain water, it effectively cuts the ratio of the nutes you had added. By now, they are possibly starving for fresh (weak, at first) nutes and micro's. Any idea whaty your ph is?
As for the lighting, I had been using a 4' x 4 bulb fluorescent shop light for veg for years, but have only ever used HPS's for flower.