Can I make my mashed potatoes around 10am to serve at 4pm? Could I put them in a crock pot on warm for that long?

Here is a traditional mashed potato recipe for the Crock Pot® or Slow Cooker. They should hold, with the occasional stirring for 1-2 hours.

Traditional "Slow Cooker" Mashed Idaho® Potatoes

That’s a very good question. The most success I have had with a slow cooker and mashed potatoes has been with the All Clad with the stainless insert. In has a very large insert so could accommodate a larger portion, such as the 16 servings you mention.  I peeled the russets, cut into even sized chunks, added to cold water and heated the insert on the stove. Once fork tender (about 15 minutes) I added a teaspoon of concentrated lemon juice to the water, pulled off the stove, drained and cooled. Covered with clear wrap and refrigerated. On Thanksgiving I mashed the potatoes, added salt and white pepper, heated it up in the slow cooker on low along with warm milk and room temperature butter, stirring every once in a while to keep from sticking. Held for about 2 ½ hours and then cooled and made mashed potato balls with stuffing the next day.

Cooking the whole batch in the slow cooker from scratch may be harder to control the potatoes from sticking or becoming gummy for the amount of time you are talking about. Can you refrigerate the cooked potatoes and then use the slow cooker to reheat? Cooking the potatoes first and then cooling and reheating is ideal, the potatoes will turn out beautifully.