The general equation governing crater size is:
D=k.E^n
Where D= crater diameter, k is a constant, E is the energy of impact, and n is a dimensionless number.
[This site describes an experiment you can conduct to determine the values of k and n:
http://helios.astro.lsa.umich.edu/Course/Labs/craters/cr_short.html]
The energy of the impact is given by
E=1/2.m.v^2
Where E=energy in joules, m=mass in kg, v = velocity in m/sec
[Because the orbit of 2004N4 is fairly well established we have a good fix on the velocity. The mass is more difficult to determine. It depends upon the average density and total volume. The 1600 megaton figure for impact energy quoted by Blobrana is based on a density of 2.6g/cc (which assumes a typical chondritic composition with little or no iron-nickel) and a diameter of about 400m. The latter is based on assumptions about the albedo - how much light is reflected by the object.)
This useful link contains Gene Shoemaker's version of the general equation:
http://www.madsci.org/posts/archives/oct98/907545968.As.r.html
The key equation becomes:
D = Sg Sp Kn W^(1/3.4)
Where Sg is a gravitational correction factor (1.0 for Earth impacts), and Sp is a density correction factor for the target material.
Plugging the values for 2004MN4 into the equation gives us a diameter of 4.6 kms, if I haven't dropped a decimal place. That should give Clockwood some nice lakefront.