The maximum width of the banner is still not enormous (how many people actually browse full screen on their desktop monitor?), but it will now be centered when the forum content below is wider. It will also now *scale* as the window gets smaller.
Having a fixed width forum (e.g., the unofficial BW forums) would make this a non-issue, but variable width is a significant advantage due to the variety of screen sizes and resolutions we're likely dealing with.
I'll look into making a banner that's a bit wider at some point -- maybe 2K -- but this is much better for now. When I do that, I'll also look at fine-tuning the background a bit more. How the forum should look to most is the background should be centered behind the forum content, and if your screen is large enough, it will fade on all 3 sides (left/right/bottom). In addition, the back should be static, as the forum content scrolls. kfreon has mentioned this isn't the case on this PC. I don't know if that's due to browser (probably) or resolution (maybe). Could be that 4K isn't playing well for some reason. Once I have time to pursue it, I'll put up a forum thread to get some info from folks on various browers and screen sizes/resolutions.