Post caps are an absolute necessity if you have deck and/or fence posts. Believe it or not, they are for more than just aesthetic reasons. I have heard plenty of folks talk about how squirrels and other critters can gnaw away at the wood posts until they look completely chewed up. Of course this not only looks bad, but it also can sometimes necessitate the replacement of the post. This will certainly not work, so I highly suggest you find some post caps that you like, and place them on your fence and deck posts.
4×4 post caps can come in a couple different sizes. I know that sounds ridiculous, but it is true for this reason: manufacturers use different standards for determining sizes. Some are nominal in nature. In this situation they are actually manufactured a half inch less. Other post caps are made with the true size. So, when you buy your 4×4 post caps, you need to understand the true size of your post and the true size of the opening of your post cap. This will save you a trip to the hardware store. If you ordered online, it will save you the hassle of shipping some post caps back.
Once you have dealt with the sizing issues, you can go to the fun part: post cap styles. There are tons of different styles to choose from, and more than a few different materials to choose from. Metal, plastic and wood are the three main materials used for making caps, and the best thing to do is choose what works best for your situation.
Once everything such as styling choice and sizing is nailed down, it becomes time to install them on your posts. Some can be drilled onto the post, while others simply need an adhesive. The manufacturer of the post cap can help with this choice.