While outwardly they look the same, this is not the case.

Each of Trek's numbered series bikes share the same frame within the series, meaning the "Four Series" 4300, 4300 Disc and 4500 all utilize the same frame. The "Six Series" frame shared by the 6000, 6500 and 6700 is a much more sophisticated frame with more internal butting and shaping, shaving a good deal of weight over the 4x series. Generally speaking the higher the number, the better the tubing and more sophisticated the engineering. This is even true with the Madone series (i.e.: The 5.1, 5.2, 5.5 and 5.9 all share the same frame, differing only in component selection). The one winger is the 820 which is not an "Eight Series" at all, but a standalone product.

There are some outward differences with the 4 and 6 series that are easier to see: the 4's seatstay wishbone consists of two stays, a flat bridge wedge and a separate connection tube to the seattube. The 6 consists of two seatstays that are joined directly to the seattube. There are other details that are tougher to see, but as far as I can tell, every tube is different except the seattube, the headtube and perhaps the chainstays, but even those could differ internally with butting or internal machining.