I have shot a LOT of Portra 160 in a wide range of lighting conditions - mostly night photography. It's a terrific film and I wouldn't hesitate to recommend it in medium format. I don't use the 400 much just because the 160 is so versatile, and when I need more light, I need a lot more light and I either throw that bad boy up on a tripod or I go to Portra 800, which is surprisingly low in grain for as fast a film as it is. Personally, I don't think you'll find the grain objectionable on any of the three emulsions (160, 400 or 800). And I have made 16x20 prints from negs shot with all three films, so I can say it does enlarge nicely.
It's funny, because I hear that Portra 400 has a lot more latitude than the 160 counterpart. I've heard people shooting Portra 400 from 100/200-1600+ in a single roll without any processing changes.