First of all, why do you call pawns chessmans? It's kind of weird @.@.... But back to the topic, I usually promote my pawn into a Queen, just like most people do, unless I want to torture him till he lose all hope of actually getting a stalemate from me and resign.
When you have a queen and one chessman goes tothe last line what does he become?

Just turn it into another queen, although you'll probably want a 3rd and 4th one too if you want to really make you're opponent squirm... Sympathy for your opponent is a weakness that must be eradicated! :-D

@quadriple
You tried it before? Or is it that you've been on the receiving end before? Which ever it is, I don't really would be so determined so through the end, especially if I have more than 1 Queen and he has nothing. A proof of this is, in most of my games, my opponent immediately resigns after they lose material ( about a minor piece or so ). This happens so often that check mating my opponent has become such a rare occurrence . So who are you to say that they wouldn't resign in an already lost endgame?

It is a choice between a Queen or Knight, depending on the situation.
Mostly a Queen is the choice.
It is a choice between any piece except a King. You could promote to a Q or underpromote to a Knight, Bishop or Rook.

when im winning in endgame and I have at least 4 pawns and my opponent only has his king I promote them all to knights and checkmate him like that =)

I know, but it is unrealistic to choose a Bishop or Rook.
Really? Plenty of reasons to not choose Q, plenty of reasons to coose a Rook instead.

It depends, usually a Queen, but it may be a Rook if i already have a Queen and the board is empty so i can avoid an stalemate.

zigwurst's point is probably that 99% of the time promoting to a queen is best, and maybe 0.9% promoting to a knight is necessary. Only 0.1% of the time (or less) will a rook or bishop be the logical choice and I would not say there are plenty of reasons. The only reason would be to avoid a stalemate.
When you have a queen and one of your chessmen goes to the last line (opponent's line) and can become: a queen, a rook, a bishop or a knight what he will become???
When i do, i prefer to do my chessman a rook, because i think it's and a bit fair for the opponent. But when my opponent has many strong pieces then i do my chessman a second queen to have more probabilities to win.