Well, indoor the season is 365 days, so 1 lb would be easy

In one grow, a lb off a 400 watter would be really tough, even for someone who has a lot of experience. The goal for a good (not average) grower is usually 1 gram per watt of light - a pound is 456 grams.

If you ran the 400watt light vertical in a cool tube or something, and could grow all around it, it should be a bit easier, but thats a lot of weight for that little light