In what part of the Cape are you staying? There are nature trails all over the Cape. With only four days I think I would forget MV. That would take an entire day.
Here is a website for nature and hiking trails. My favorite is the Atlantic White Cedar Swamp in Wellfleet.
You should also visit the National Seashore Visitor Center in Eastham.
Provincetown is also well worth a visit as is Wellfleet:
Do you have reservations yet? It seems like you may not since you're asking about B&Bs. If not, you need to make a reservation ASAP. I always suggest staying mid cape. That way you can travel around the cape and not be too far from anywhere. Provincetown is a must see for sure. If you head up there, you can stop in Truro on the way at the winery there for a tour and tasting. The cedar swamp is very pretty. If you want a bike ride you could do the cape cod rail trail which starts in Dennis or you could do the Shining sea bike trail which starts in Woods hole. As for restaurants, we can give you suggestions depending on which part of the cape you stay. Also let us know what kind of food and price range.
I am not being mean and love to answer questions but you have to take a little responsibility to look at prior posts and find sites of the easily found information like bike trails etc. Every one of your questions has been answered.
MV personal choice. Some people go others don't. Both sets of people have great times so without some context, there is no answer for you.
You can rent bikes and ride the Cape Cod Rail Trail. We have rented from Brewster bikes and Barb's. Both are located on the bike trail and have free parking. With a short stay, I would pick Cape Cod or MV, not both. I don't stay in B and Bs as we prefer a heated pool on our trips to both places. Chatham fish pier is a must see for us to see fish being unloaded, view seals, and for lunch.
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