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A typical 4th axis is a rotary table/ spin index. As far as "parts to buy", you could buy a complete assembly like this : Weiss TC 150T Rotary Index Table | eBay or you could build one from scratch or you could convert an existing manual table/index. A great deal depends on your machine and requirements. There is a HUGE difference between a "mill" and a "router machine". A router based machine implies you are working primarily in wood, and "medium sized" is almost meaningless as typical wood working machines range from a 6" x 6" work envelope to 5ft x 12ft work envelope. A mill typically implies a machine capable of machining metals and is typically made from cast iron and steel. While mills range from "desk top" sizes with 6" x 8" work envelopes to huge industrial machines, a very common size is the Bridgeport style mill which typically has a work envelope of ~9" x 42".
Things to consider in a rotary table/spin index wrt your mill or router: 1) Weight 2) Height 3) Accuracy 4) Intended use 5) budget. Since it does not appear you have a specific use in mind for a 4th axis as of yet, I would urge you to focus on your machine until the need arises for a 4th axis.