It depends how picky the person is on color accuracy and how their BULB ages and how many hours per year they put on a projector. No-one can say adamantly either way as it is borderline but just under what I'd call the absolute max usable. I would still recommend gain at this screen size even though it is possible to go without. Every lamp dims at different rates, people prefer different levels of brightness, etc... The standard is 14 fL, which means eventually you'll need to crank the lamp into high mode sooner than later to maintain the standard. There are a lot of factors, generally 120" for the hc4000 is pushing it to the edge of its limits as the lamp ages, and therefore screen gain is recommended.
If you choose to not do gain, all that really means is you will not get as much "bright usage" out of the lamp as a smaller screen and you may have to go out of BEST mode sooner. Overall, this will increase the cost of your lamps and make it so you have to watch with less accurate color more often than on a smaller screen. The brighter the merrier, and rarely do people find they have too much gain, most of the time they do not have enough. Overall I'd recommend some gain for 120" with this projector if you can, but you can get by at this screen size without it if you don't mind replacing lamps more often.
Edited by coderguy - 8/7/12 at 10:11am