We use ellipsis points to indicate that we've left out words, usually from another piece of writing or speech that we are quoting. Ellipsis points are three periods, with a space before and after them, thusly . . . and they mark omissions.
The Merriam-Webster Dictionary ) defines them:
1 a: the omission of one or more words that are obviously understood but that must be supplied to make a construction grammatically complete
b: a sudden leap from one topic to another
2: marks or a mark (as ...) indicating an omission (as of words) or a pause
This is pretty clear. So what's the deal with the three and four dots? When one and when the other? The answer is . . . one of the four is not an ellipsis point at all!
In decades past,The Chicago Manual of Style (13th and 14th editions)
recommended that everyone observe the difference between three- and four-dot ellipsis. Three dots indicate missing words that are omitted from inside a sentence. Four dots indicate a whole sentence or more than one whole sentence has been omitted. WHY? Well, actually, the first of the four dots is actually not an ellipsis point at all but a period at the end of the previous full sentence.
So all ellipsis are always three dots. Sometimes they are preceded by a period ending a sentence right before the ellipsis. Voilá!
That period should be closed up tight to the word before it, just like a regular period. Then an ellipsis. Simple, eh?
But today, the Chicago Manual of Style 15th edition acknowledges that this simple rule may be too much trouble to observe. It recommends three levels of observance, which I've graded:
Simple-can be observed by anyone: Always use three dots when you leave something out. Don't worry about how to indicate full sentences or not.
Professional-observed by most editors and authors: Use three or four dots, depending on whether the preceding is a full sentence or not.
Rigorous-observed by textual scholars and legal writers: Use three or four dots, and if you change the capitalization of something because you left something out, put the newly capitalized or lowercased letter in brackets to indicate a change from the original.
And of course, much more important than how many dots is fidelity to the original meaning.
Much as an editor might want to edit the quote: "This book is a cesspool of uninformed and startling opinion. It's too bad such drivel is published about one of the most important issues of our time," one must not edit it to read "This book is . . . startling. . . . about one of the most important issues of our time," no matter how correctly the ellipsis are used.
Of course, if any readers have great stories about ellipsis like that, we'd love to read them. Elide the publisher's name, the author's name, the employers' names. Elide away, but do tell!