It sounds like one or both walls slope outward at the top or are bowed out in the center. There are various options, depending on how particular you might be. First, find out whether you are dealing with one or two angled walls. If both are sloped, the angle on each side may be minimal. A quarter inch gap over four feet isn't that noticeable, unless you are studying the flaws, which only the person who does the construction will every know. Everyone else just looks at the final results and will be impressed. Installing corner strip molding or quarter round can minimize the visual effect. If you plan on doing this, start by installing the first panel centered in the wall, with the edges at studs, shimmed up from the floor and level. When you get to the corner, push a piece of scrap beadboard up against the corner and use a compass (drafting, not directional) to mark a line equidistant from the corner. Cut on this line and use it as a template to cut the final piece on each side. If possible, make sure that the closest vertical line is as far from the corner as possible. If you are a perfectionist and don't want any angle to show, either get new beadboard that doesn't have vertical lines, put unlined wallpaper on your beadboard or install strips at an angle, instead of beadboard. You can install them all at 45 degrees in one direction or change directions in the center for an arrow appearance or get fancier with a diamond or multiple diamond appearance. You can use double stick tape or putty to see what it will look like, before doing your permanent install.