Actually, thinking about this, you already have 4x100. Wheels etc for this are a lot easier to find than 4x108. For example, Golf discs in 256mm are available. The main thing is mounting the calipers. There are sure to be aftermarket adapters aval to go from 239 to 256mm discs? This is a well-travelled route with the Mk2 Golf crowd. See
www.mateymatey.com
IMHO it is not worth changing hubs etc to 4x108 simply for the sake of wheels you have. Especially if you decide to stay with drums on the back. It is better to get 14" wheels in 4x100 and put 256mm Golf discs on the front and remount your calipers.