@jamesdhatch posted a photo-link to a blast gate. eflyguy posted a picture of a back draft prevention damper. The Louvered Vent Cap is a popular option and is found at Home Depot, Lowes, Menards, etc… as it is meant for the end of a dryer vent (which is also a four inch hose.) Here is a picture:
You’ll find that picture in @marmak3261’s post about his DIY magnetic quick disconnect coupler.
There seems to be a rumor that you can buy one, but I didn’t find any links to a commercial one in my very quick search.