Well, uha, well, its not quite that simple as a 1:1 of Carapils to maltodextrin and they are not exactly the same although the final affect on the beer is similar.
Maltodextrin is a product made from starch that is souble in water and is added to the kettle to add body, mouthfeel, and foam stability to the beer. Based on its effects, it is much more concentrated than CaraPils. So 4 oz of maltodextrin has a greater effect on the beer than 4 oz of CaraPils.
CaraPils is undermodified barley malt which contains high levels of beta glucans and a small amount of dextrin or maltodextrin. It is added into the mash contains proteins, tannins, etc unlike maltodextrin. It is dried to have a low color of 1.2 - 1.8 L and minimal flavor but it is not completely tastless. It is likely the body, mouthfeel and foam stability comes form the thickening properties of the beta glucan. The effects of CaraPils on thickening beer is limited compared to maltodextrin. If you use up to the recommended 5% of CaraPils and want additional body, you need to change to maltodextrin.
If it were me following the recipe, I would use maltodextrin as suggested. If you like the beer and brew it more often, experiment with a beer made with the max (5% or so) of CaraPils.
Dr Malt