You can always get more out of your marriage and anniversary gifts if you include more than one theme to the gift. If your family loves traditional and symbolism based gifts, then you could add any or all of those themes to the chosen 4th anniversary gift to send them a symbol that will be a lasting memento. If the theme is something more personal to the couple's relationship, such as sports, a hobby, or anything that they have in common, they'll appreciate it even more.
A wedding anniversary, unlike most anniversaries, doesn't have a single theme. For example, a couple might get married in a very traditional church with a large flower arrangement at the reception. However, it would not be surprising to find that one of the groomsmen has taken the idea of the whole wedding and just included it into the groomsmen gift. The groomsmen gifts could be as simple as a personalized golf towel or golf shoe, but he could also include a t-shirt with a picture of the bride and groom in it, or perhaps a tumbler with an image of their family on it.
Another idea for a gift would be to give the couple a great sports theme to select from. For example, some couples like to go skiing and they might get together to ski together. A great gift for the couple in this case would be a ski trip package for two or a ski pass for two.
Couples like to go on a vacation. A great gift for the couple would be a package for two to the beach, or to a place where they can enjoy some kind of outdoor activity like hiking. If you're not sure if the couple really wants to go to the beach, maybe you could give them some tickets to a concert, or a concert ticket and tickets to see the band they love. A great gift for couples like this would be to buy them a concert ticket and tickets for two.
A couple's interest can vary, so it's important to get a gift that fits their interests. One gift idea would be to buy them an eBook on a hobby they have and that they will use for years to come, such as bird watching, sewing or pottery making, for example. A great gift for couples like this is a gift certificate for a few hours on a sailboat. For a couple who loves nature, a good idea for a gift is a book on wildlife and natural history, or a guide to camping.
If there's a specific type of gift you want to give that's not listed above, try to think outside the box. Don't hesitate to send them something completely unique, and you might find that you have some great ideas for the couple. It's always a good idea to make sure to ask if the couple likes the gift first to help keep it from getting lost in all the different gift categories!