In lighting there is a saying " You can never have too much power ". So it really is down to what you can afford. The price difference of £100 between the D light 2 and 4 buys you one more available stop of light. If you are thinking of groups then that could be valuable from a dof point of view.
Specs wise both D lights are adjustable over a 5 stop range. Reclyle times are 0.7 secs. The lower powered D-Light 2 gives a 1/1200 sec flash duration and the D-Light 4 gives 1/800 sec duration.
Ignore flash duration as it is immaterial for what you are going to do.
So in the end if you can run to a D-Light 4 then thats the one to go for.
Link to Elinchrom web site
http://www.elinchrom.com/
Don