In Catan, how do you decide which "fifth" resource to neglect?
If you must neglect one of the resources, then there are several possibilities, the main three  are:

Neglect Ore.  Roads and settlements do not cost ore, thus it is possible to start without ore and build a settlement on a ore hex later.
Focus on Ore, Wheat &amp; sheep, going for fast 2 cities and development cards, then with the extra resource production trade your way into another settlement.
Harbor 2:1 + Go heavy on a specific resource.

The best option depends on many aspects including:

The distribution of resource: abundant resource goes down in importance, scarce resource goes up.
Trade possibilities: either with the other players or with the bank. Does another player needs your resources? How trade-happy are your opponents?  Do they think you are a head (which will make them less inclined to trade with you)