A note on 'resentment'
The Oxford English Dictionary definition of the verb resent:
7. trans. To feel oneself injured or insulted by (some act or conduct on the part of another); to show that one is displeased or angry at (some wrong, injury, etc. sustained).
A 'resentment', therefore is any time you feel injured, insulted, displeased, or angry.
The etymology (word history), contrary to what you will hear in meetings, is not 'to feel again' (from the Latin re, meaning 'again', and the Latin sentire, meaning 'feel'). The sentire part is correct; the re, however, has the sense 'in response to a stimulus, with intensive force'. Note that the prefix re has many different senses.
You could therefore understand 'to resent', for the purposes of your Step Four, to mean 'to feel intensely in response to a stimulus'. The Big Book (pages 64:3 and 65:1) uses the following 'trigger words':
resentfulangryhurtthreatenedsoreburned upgrudgeinjuriesa sense of being interfered with
When the text uses the word 'resentment', take that to include any of these emotional states or responses. The negative feeling need not relate to past events only. The 're-feel' explanation is not entirely harmless in an AA context, as I've heard the argument voiced that it's normal or healthy to react in very strong ways to the events of day-to-day life and that all we need to inventory is 're-felt' emotions, i.e. resentment at past injuries and so on. This is to miss a great chunk of what separates us from God and can actually cause the bulk of our difficulties in everyday life: how we're reacting unhelpfully to people and situations around us in real time.