I'm renting out part (a studio room) of my house. Do I use the full house value to calculate original purchase price, fair market value, land value, etc.?

[swearmir]

For example, if I purchased a 4-bedroom house for 400k, do I value the room at 400k / 4 rooms = $100k?

[AnnetteB6]

Yes, you use the full value and the proportion of the value based on the space that is rented.  

You can use a method based on the number of rooms like you described in your example.  Typically a more accurate calculation would be based on the square feet of the space being rented versus the square feet of the whole house.