How long do we cook four large rutabaga?

Or roast 40 minutes at 425F (cut in large chunks or wedges, tossed with a little bit of olive oil, salt & pepper, spread on a sheet pan or large cast iron skillet).

Do you have a plan for how you're using them? That might impact how you want to cook them. If you haven't settled what you're doing with them yet, I like the method found in this recipe, in which they get simmered in wine, vinegar, spices, and aromatics: https://food52.com/recipes/25768-rutabaga-a-la-greque for 15 minutes (perhaps longer with 4 large ones though) or until tender.