yes, you could veg under w/e light schedule you'd like, so long as they don't receive enough dark to build up the flowering hormones which would make them start flowering..
also, plants and sugars are made during photosynthesis, so i think more light is better, but only to a point.. i once read jorge cervantes say that if you veg your plants under 24 hours of light that they will grow 33% faster than lights grown under only 18 hours of light..
so of course after reading this, i thought to myself, shit, i want my plants to grow 33% faster, and through mine into 24 instead of 18 / 6 or 20 / 4... it seemed to me that my plants actually didn't grow any faster under 24, and in fact seemed to be doing a bit worse imo.. not that they looked bad, but it was very evident that they were doing much better when they had some dark time.. whether it's 4 hours or 6, i don't think it really mattered too much, but ime, some dark is better in veg than no dark at all..