Usually bonuses of the same type don't stack
The System Reference Document on Stacking says
In most cases, modifiers to a given check or roll stack (combine for a cumulative effect) if they come from different sources and have different types (or no type at all), but do not stack if they have the same type or come from the same source (such as the same spell cast twice in succession). If the modifiers to a particular roll do not stack, only the best bonus and worst penalty applies. Dodge bonuses and circumstance bonuses however, do stack with one another unless otherwise specified.
An enhancement bonus is a type of bonus. Thus, for example, a fighter wearing gauntlets of ogre power (that grant a +2 enhancement bonus to the fighter's Strength score) who then is the target of the spell bull's strength (that grants a +4 enhancement bonus to the fighter's Strength score) receives only the higher bonus, that provided by the spell bull's strength.
Exceptions exist, of course, but enhancement bonuses aren't one of those exceptions.