So you can attach a dr4b to Most drivebases. I use the longest one the 35. So 35ling and 35 wide. If you look in the search bar for quality drive you will find some really nice ones for drives at least set up by @Xenon27 also for your dr4b look at 333a double reverse bar tutorial also in the search bar walks you by step-by-step.
A shoulder screw is a screw with a shoulder in place of the threads at the top of the screw. This shoulder is just a little less wide than the holes in the vex metal. This makes it so any 2 pieces of metal you screw together will be aligned in coordination with that hole and you won’t have to do it. A box bolt is when you take a long screw and put it through a c channel to the other side with spacers on it. So it stops the c channel from conforming inward or outward, thus giving it more strength.