Well comparing to say WW & SW I seem to recall that on bith of those the bigger you are to start the more points/syns you get, then as you reduce so do they.
That would suggest to me that you would be best starting with 4 and seeing how you get on.
If you're not losing the weight you want to at the speed you want to after the first pack (which will be about 3 weeks) then drop to 3. If the weight loss is going OK and you feel OK on 4 stick with that for a while.