Nice LR4 umbertob!
I think the G4 decal would look really good on the rear, go for it!
I have been pondering the black vents on my santorini black lr, really want the black door handles!
I love my current G4 decals, but I only have them on the sides under the indicator lights. The official instructions/templates have them going in that location and the rear, so I think I will go ahead and try it out when I get my new 2013 LR4.
Here is a great source for grills, vents, etc. You can also look on eBay - they have the vents for sale. I actually painted mine - it is pretty simple is you use a primer/paint combination - sticks quite well.
Here is a great place for accessories and reasonably priced:
http://www.lrparts.net/
Also, I know someone who bought these and were really happy:
http://www.adamesh4x4.co.uk/body-styling-116-c.asp
The door handles are the ONLY thing not black on my current 2011 LR4 (I did all the black trim everywhere else), but I will leave as is since I expect my 2013 LR4 (with black design package) to be delivered in a couple months.
-David