IMO you won't be overskimming that much with a G4. They overate their skimmers. It is only about the same size as a euro-reef CS8-2, which is rated for 155 Gallons. As mentioned already, many people use big skimmers on smaller tanks, I think the only draw back is you might have is a nice tank!
As a note, I am planning on getting a G4X for the 155G I am setting up. The guy I am buying it from runs a G5 on his 90G SPS tank. If I had money to blow I'd like to get the G6! Nothing like a 4 foot needle wheel skimmer
