I believe you've got the distinction between predicating and illocutionary act a little bit off.
Some historical background. In Aristotle's On Interpretation, he addresses the question of how complex meanings are built up out of simple, uncombined meanings. For example, 'human' is a simple meaning without combination, whereas 'humans have legs' is a combined complex expression. Not all combined expressions make a meaningful whole: 'green loudly', for example, fails to have any unified meaning. Not all apparently complex expressions are actually complex: 'pineapple' appears to be a combination of 'pine' and 'apple', but this is in fact a simple expression. Moreover, some simple expressions are meaningful by themselves ('human' refers to something int he world), while other simple expressions ('is' 'will be') are not meaningful apart from combination. Aristotle;'s theory of how to parse these distinctions and how complex meanings are built up out of simple meanings in a rule-governed way is his theory of Predication.
Ok, so predication is the combination of simple meanings into complex meanings. Now what about illocutionary acts? Aristotle had failed to distinguish between predicating and asserting. So predication comes for Aristotle to play two incompatible roles: the role of building up complex meanings out of simple ones, and the role of judgment: i.e., taking a position on the truth-value of the complex meaning.
However, notice that predication can occur without judgment: such as when I ask, "Does Sam smoke habitually?" This predication is the same predication as if I were to state to you: "Same smokes habitually." That is, it is the same complex meaning made up of the same simple parts. But it doesn't do the same thing in conversation. It has a different illocutionary force. We need to have two separate concepts here: predication, on the one hand, for putting meanings together into further meanings; and illocutionary acts, on the other hand, to capture the attitude we take toward the resulting complex meaning.
Searle's point, ultimately, is that the attitude you take (judging to be true, hoping to be true, fearing to be true, wondering whether true) is not part of the propositional meaning of the proposition, but part of the attitude you take toward it. The propositional meaning of This is Sam and Is this Sam? are the same; the illocutionary force (assertion in one case; questioning in the other) is what accounts for the difference. If you look at your list 1.-4., notice that there is both an element of sameness and one of difference in each of the sentences. The distinction between attitude and proposition is Searle's attempt to capture that element of sameness (sameness of proposition) and difference (difference of attitude).
A complete speech act, then, is a complex act that consists of two sub-acts: an act of putting-together of a propositional meaning (predication) plus an act taking an attitude toward that propositional meaning. Neither a proposition by itself nor an attitude by itself does a complete speech act make.
In one of your comments, you ask:
let's say there is a new employee to the company, and my boss says to me while pointing at the employee 'This is Sam.' In this case, there is clearly a reference to that person. Is this act of referring a propositional act, or an illocutionary act, or both?
This is a complete speech act, which means it has both a predication and an illucutionary act. Here your boss is doing a predication: he is putting together the meaing 'This is Sam' from the simple parts 'this' 'is' and 'Sam.' He is also doing a separate illocutionary act: he is asserting as true that 'This is Sam.' He could also perform the same predication with a different illocutionary act: "Is this Sam?" Same predication (he is yet again putting together 'this' 'is' and 'Sam'), but this time he is not asserting this as true. Rather, the illocutionary act he performs is one of questioning or wondering. In both cases, the boss puts together the same meaning (performs the same predication). But he floats that meaning over to you in conversation in a different way (performs a different illocutionary act). You can also take other attitudes to predications, resulting in different illocutionary acts: hoping, regretting, fearing, etc.