(bold means a verb, italic means a noun, parentheses mean a prepositional phrase, strikethrough means an adjective).
"Four (plus two) is
equal(to six)" N-LV-A
"Four (plus two) is six." N-LV-N
"Four (plus two) equals six". N-LV-N
In the first sentence, "is" is a linking verb or copula. Here "equal" is an adjective, and the predicate of the sentence. The word "six" is the object of the prepositional phrase "to six", which functions as an adverb, modifying the word "equal".
In the second sentence, "is" is still a linking verb, but instead of an adjective, "six" is now the predicate of the sentence, which circumvents the necessity for a prepositional phrase.
Finally, the third sentence does not use "is" at all, but instead uses "equals" as its linking verb. It is more or less identical to the second sentence, but replaces "is" with a more colorful, more descriptive linking verb.
As a somewhat loose follower of the E-Prime philosophy--which in its strictest form mandates the omission of the verb "to be" and all of its conjugations--I would be most inclined to use sentence 3. A quote from the Wikipedia article on the use of "equals" rather than "is":
Replacing statements including "to be" with those using becomes, remains and equals divides perception of, and expressions about, time more operationally into actual cognitive categories that humans know how to act upon.
To claim that one thing equals another is a claim only about the present with no reference to the future or the past—it can be disproved by direct testing.
