chlorine is highly reactive and thus reacts faster than bromine. fluorine is even more reactive (too much) and this is why we don't see it in free radical halogenation. on the flip end, iodine is unreactive due to the unfavorable thermodynamics (it's friggin slow).
for mcat purposes, faster reaction by chlorine equates to less selectivity
Referring to the first propagation step (halogen radical removes a hydrogen from a carbon to create a carbon-radical):
If you look at a table of bond enthalpies, you'll see that a chlorine-hydrogen bond is much more stable than a bromine-hydrogen bond. Creating a chlorine-hydrogen bond is therefore much more exothermic than creating a bromine-hydrogen bond. Since more energy is retained by the formation of a H-Cl bond, creating a less stable carbon radical isn't as much as a concern since the energy lost by breaking most any C-H bond to form the radical is easily regained when the H-Cl bond is formed.
With a bromine-hydrogen bond however, the system must conserve as much energy as possible since much less energy is gained by the formation of this bond. The most stable carbon radical is therefore energetically preferred in a bromination.
The same trends can be applied to I and F. Just look at a table of bond enthalpies and compare the energy needed to break a variety of C-H bonds to form the carbon radical, and the energy created by forming the hydrogen-halogen bond. In general when the H-X bond formation is very exothermic, the system doesn't really prefer one radical over another since the additional energy needed to make a less stable radical is more than returned. This trend explains both the speed of radical chlorination relative to bromination, as well as its lack of specificity. Hope this helps.