Catan's initial setup follows a snake draft structure, so the strategies available to the players in different positions are fairly different. As a general rule, the players at the beginning of the settlement placement should focus on the raw production numbers of their settlements while the players towards the end should focus on specific resource combinations.
In the first or second seat, your first settlement will likely have good production numbers and/or good resource combinations. On almost every setup, there will one or two hexes of a fairly rare resource (like brick or ore) with decent production numbers (5, 6, 8, 9). Only one or two of those locations will have good neighboring hexes (a
5, 8, 10 location is much better than a
8, 10, 11). This means that the first few players will likely get the "best" single settlement locations. However, the second settlements will likely be a bunch of leftovers. It's hard to plan for what will be left for the second settlement, so focusing on the best first settlement is the main strategy.
In the fourth seat, the most abundant locations will likely already be taken. However, you get to play your settlements back-to-back. This means that you can choose the best synergy between your two settlements. While your raw resource production will likely be not as good as those with the "best" locations, you'll probably have an easier time getting resource diversity. Strategies like trying to get a 2:1 trader and placing your other settlement on an excellent production location are possible.
Third seat can be awkward to play, but it drafts more like fourth seat than the first two. You just don't get the back-to-back placement, so it can be hard to set up more specialized strategies.