If you're happy with the 2m and it's for general traction kiting or just static flying then the 4m will be fine.
Probably plenty of deals on a Buster II at the moment.
I flew a 2m when I started ... great fun in high winds

Lots of the folks down our spot started with Busters and no one has a bad word to say about them. Good build, good VFM ...... the handles are like 'Marmite' (you either love or hate them) ........... not a great fan of the lines (purely because mine broke and I prefer Ozone).
Lots of Buster II reviews here .....
http://www.racekites.com/reviews/listReviews.asp?companyName=PKD
Happy hunting.