There are 13 possible tricks per hand in Spades... but due to a site anomaly, a maximum of 12 or 14 tricks can be bid. For argument's sake, let's just use 12 tricks as the max. Ideally, player #4 has an early advantage as he can sit back and observe what the other three opponents bid, i.e. how strong or weak their hands are (also, who has which) and can use this information accordingly. Before starting each hand of Spades, all players count up the quality of their cards. Here is a way to find the quality:

-Count the number of spades you were dealt. In a deck, there are 13 spades. Divide it into four hands, and an average hand has about three spades. Add one point each if your spade is Queen, King or Ace.
-Add one point if you have four spades, two points for five spades, three points for six spades, etc. This additional spade(s) will be used as a cutting card or a stand-alone spade after all the other spades have been drained from your opponents.

NON-SPADES
-Add one point for each Ace (if you have five or more cards in the same suit as the Ace, add only a half point because your Ace now has a 50-50 chance of getting cut).
-Add one point if you only have one or zero cards for a particular suit. Now you can use a spade to cut early (do NOT count it if you've already counted 1 point for having four or more spades. If you do, then you are counting the same point twice which may result in you overbidding your hand).
-Add half a point for each King (unless you have four or more cards in that suit. If you do have four cards or more, then your King will probably get cut).
*****

Convert all points into tricks (one trick for each point), and consider whether you want to count the half-points or not. The half-points are possible tricks. If you are the fourth bidder, you can use the opponents' bids as a guide, i.e. you counted three points and two half-points for your hand. You're bidder #4 and see that each of your opponents have bid three tricks each. In that case, don't count the two half-points, just bid the three tricks (you can't bid for the 13th trick even if you wanted to. Site anomaly, remember?). But if your two possible tricks work, then one of your opponents is going to be set.

If you count only 0-2 points, you might consider bidding nil... but that's a whole different story on it's own.