Run out on your bandsaw is not completely controllable but you can eliminate alot of it. Use the widest blade you can install(not thick, Wide). Use the highest quality blade you can find, typically this will be mail order. Home building supply shops simply don't carry the level of qaulity needed. The high quality is to find highly balanced teeth, correctly and highly accurately sharpened. Teeth cutting too aggressively or too lightly on one side of the blade or other will run the cut in that direction regardless of how much tension, in fact too much tension can worsen the effect. Also, a fresh, quality blade out of the package will be sharp and ready to cut straight, you can sharpen an old blade but you'll run through alot of wood before you get all of the teeth balanced in their cut. At 4 inches I don't think blade guides are going to add much but you can surely add good quality guides to help the blades orientation as it enters the wood(lessens the effect grain will have on the run of the blade but only in the first inch or so, by far, teeth orientation, offset, balance, sharpness will make the difference in runout).