I love all four of these things. You can imagine them as a series of overlapping circles- a Venn diagram of abuse.
Identifying them is pretty easy once you get the hang of it.
Degradation involves eliminating self-esteem, pride or status. It is the largest of the circles in this diagram. Basically all other actions in this sphere contain elements of degradation.
Humiliation typically involves other people and lowered standing amongst peers. It doesn't necessarily involve lowering of status and often feeds of the dichotomy between status and action- but often part of the pleasure is the idea of one's own loss of self-esteem.
Dehumanization involves ignoring essential human qualities and treating the submissive as if she did not have them. Pet play fits in here- eg. making a submissive scratch at the door if she wishes to be "let out" to use the washroom (the backyard).
Objectification is a form of dehumanization wherein the submissive is reduced in complexity and meaning to something very simple. Terms like "cumdump", meant to get across the idea that the submissive is simply a receptacle, generally are used in this way. In fact, receptaclization is a term I use to denote an even more specific subset wherein a submissive is used as a garbage pail or toilet. More innocently, this can also refer to submissives being used as furniture. It can also mean exactly what it means when used in regular society- the woman is seen as a purely sexual object with little or no other utility.
-IC