Three weeks from veg to 12-12

My plants are four weeks from seed.about seven inchs tall.short and bushy.using 400 watt mh for veg and 400 hps for flower.space is an issue.some type of indica strain(bagseed) and I wanted to no could I switch to 12 12now.oh growing six plants

sure, go for it.

You can go to 12/12 now, you will still have some "veg" time left as it transitions to flowering over the next 2 weeks, expect your plants to at least double if not triple in size from the point you switch to 12/12.