It is not all that simple, you have to know if the part you are working with is spec'd in OD or ID. Pipe size is NOT all the same, USUALLY OD, but not always, fittings can be OD or ID, you have to ask. Some companies specify thier components in OD, some in ID, some in OD/ID. I had to do some custom exhaust plumbing on my Deuce, it took a lot of time at the parts store looking through the parts books figuring what combinations of IDs and ODs and adapters I needed to get everything to fit. Some parts were OD/OD, some were ID/ID, some were OD/ID. You have to look at the parts book and see if the part you want is OD/OD, ID/ID or OD/ID and plan your job accordingly.
Simply example. You want to buy an elbow to mount your air intake mushroom vertically to minimize intake of rain/snow. The air inlet is 4", so you buy a 4" elbow, right? WRONG, a standard 4" elbow is 4" OD x 4" OD. the air inlet is 4" OD, the mushroom cap is 4" ID, so you need a 4" OD x 4" ID elbow. The 4" ID side of the elbow fit OVER the air filter inlet, the 4" ID side of the elbow fits INSIDE mushroom cap. Not rocket science, you just have to think about what you are trying to fit together.