In each post, there is a little box on the right side, under the body, but above the signature (if any). When the post is first made, the box is grey and contains the number zero (0).
Other members can also see a green square to the left of that box, with an arrow in it pointing up. If a member wishes to, they can click the box with the arrow to express their approval. Doing so will increase the number in the box, and the box will turn green showing the number of votes. A member can only vote once per post, and never on their own posts.
Each vote is counted twice: once on the post where it is made, and once in the profile of the person who made that post. That counter is the member's reputation.
So, in your case, four plus votes have been made. They could all be on one post, or could be spread around.