1, 2, 4. Guess, guess at, conjecture, surmise imply attempting to form an opinion as to the probable. To guess is to risk an opinion regarding something one does not know about, or, wholly or partly by chance, to arrive at the correct answer to a question: to guess the outcome of a game. Guess at implies more haphazard or random guessing: to guess at the solution of a crime. To conjecture is to make inferences in the absence of sufficient evidence to establish certainty: to conjecture the circumstances of the crime. Surmise implies making an intuitive conjecture that may or may not be correct: to surmise the motives that led to it.

Did you mean guesstimate?