You have to serve your guests some type of food and the type and amount of food you serve is determined by the length of time your reception is as well as the formality of your wedding.
It is perfectly acceptable to serve brunch, to offer one main option instead of three or limit the bar. You can also decide to have a cocktail food only or a dessert only reception but then you should limit your reception time from 4-6 hours to about 3 hours.
Your guests will expect to be served food and probably a full meal. A friend recently told me that she went to a wedding that was from 6pm to midnight and there was only a limited bar and some small finger foods served over the course of the six-hour wedding. Finally, out of hunger, a guest ordered pizzas and had them delivered to the wedding and he paid for them.
As for formality, the more formal your wedding the more involved your menu will be with multiple course and a cocktail hour too.
As for the type of food you serve that’s entirely up to you however I always recommend adding pigs-in-a-blanket to your cocktail hour menu.