In CRM 3.0, following were the events against which we could have written our callouts
Pre/Post – Create, Update, Delete, Assign, SetState, Merge.
In CRM 4.0, the above event are now referred as messages for plug-ins. All the above events that were in CRM 3.0 are there as well as there many new messages/events have been added against which plug-ins can be attached.
For e.g. we can attach a plug-in against AddMembers message for team entity which would fire when we are adding members to a team.or attach a plug-in against Retrieve message for say lead entity which would fire when retrieve method of CrmService is used.
We can find all the new messages and the entities on which they work in SDK.