Although on rare occasions it might be for the number of goals a favored team is projected to win by, in the overwhelming majority of cases if you see 4.5 on a soccer betting line it is referring to the over/under for the total number of goals that will be scored in a game.
The way it works is simple with a 4.5 line in soccer. If you take the over and five or more goals are scored, you are a winner. If you take the under and four or fewer goals are scored, you are a winner. The exact opposite holds true for the losing bets. If you wager on the over and only four goals are scored, then you have made a losing wager. If you take the under and the teams combine to score five goals or more, you have also made a losing bet.
The reason you see a .5 after the whole number is so the bet will always have a winner or loser. If it's just a whole number and the teams combine to score exactly the designated amount of goals, then the bet is considered a push (or a tie). In this case, you get your money back as you have neither won or lost the bet.
It's unusual to see the 4.5 number on an over/under for a soccer bet. Although there are plenty of variables such as the quality of the play, the average soccer game ends with between two to four goals. In the last five years in the English Premier League, for example, the average was 2.73 per game.
There are other factors to take into account as well, not the least of which is a team currently on a hot streak in terms of scoring goals or has a goalie who is doing very well at recording shutouts, which are often refered to as clean sheets. Another variable is a team may be in the position where it does not need to score multiple goals or even score at all. This can come into play in a home-and-series in knockout play where the final combined score determines the winner. As such, a team that won the first game by the score of 3-0 might come out in a defensive posture knowing it can afford to play for a tie or even lose the rematch 1-0 or 2-0.
As mentioned above, 4.5 also could refer to a case in which bookmakers are predicting the final margin of victory for the favored team. Considering the average number of goals scored in a game isn't even 4, then it should come as no surprise you would hardly ever see a team favored by 4.5.
The potential exception is when a vastly superior team plays a team that is vastly inferior. This would be most likely to happen in a tournament such as the FA Cup, which is open to any team in England and includes teams from the Premier League right on down to the bottom (level 10) of the
England football hierarchy. In the 2011-12 season, a record 763 teams entered the FA Cup. One of the top teams in England could easily beat one of the lowest by five goals, but even this could be tricky to predict because the favorite might not use its best players or take to the field with any sense of urgency.